A famous example is the Treaty of Wuchale, between Ethiopia and Italy in 1889. In the treaty, the translation of a single disputed word would change the treaty's meaning, and this disputed meaning led to a war.

Background

The 1880s were the height of the European Scramble for Africa, and Italy had focused its colonial ambitions on the Horn of Africa. Eritrea was a territory with unclear sovereignty - many of the region's ports had previously been controlled by the Ottoman Empire, and then the Ottoman vassal state of Egypt, but local rulers and prominent merchants also had de facto control of many ports. The interior was controlled largely by Ethiopia, but Ethiopia at the time was highly decentralized, with local rulers largely exercising actual control of the Eritrean interior. Italian businessmen had purchased various ports on the Red Sea coast in what is today Eritrea as early as 1869. After a failed Egyptian invasion of Ethiopia, revolts in Egyptian-controlled Sudan, and general financial woes in Cairo, Italy took over Egyptian possessions and by the mid 1880s controlled much of the Eritrean coastline. Italy was looking to expand their control inland from Eritrea, and skirmished with the Ethiopians in battles over disputed inland territory

But in 1889, Ethiopia's Emperor Yohannis was killed in battle against Sudanese forces, and a brief civil war broke out over the succession to the Ethiopian throne by rival feudal lords. The loser of this fight was initially backed by Italy, and to end Italian support for that pretender, the winner of the civil war - the Emperor Menelik II - agreed to make a treaty with Italy to secure his throne. The treaty was named after the town it was signed in - Wuchale

The Treaty

Most of the treaty is of the standard "unequal treaty" terms - the Eritrean-Ethiopian border was outlined, Ethiopia recognized Italian colonial control of Eritrea, Italian citizens were to have special rights in Ethiopia and be judged by Italian laws, and Italy would be the favored party in any dispute or contract where Ethiopia was deciding between Italy or another country, etc. Two versions of the treaty were created, one in Italian, one in Amharic

The Disputed Translation

The key dispute was over Article 17 of the treaty - the clause outlined below (in english), emphasis mine

His Majesty the King of Kings of Ethiopia can use the Government of His Majesty the King of Italy for all treatments that did business with other powers or governments.

In the Amharic version of the treaty, the can in question has a similar meaning to English - Ethiopia has the option of using Italy as a conduit for relations with other countries, but it is not obligated to do so. In this reading, it is consistent with the rest of the treaty as a type of "unequal treaty" - Ethiopia is clearly the subordinate partner in this treaty, but Ethiopia remains independent and can conduct foreign policy on its own

However, the Italians claimed that in their version of the treaty, the can in question has an obligative connotation - essentially reading in English as the Ethiopian king must conduct its foreign policy through Italian auspices - essentially turning Ethiopia into an Italian protectorate, an unfree subordinate state where Italy controlled the country's foreign policy and leaving it only limited internal autonomy

The Impact of the Dispute

It might seem like a minor change, but this translation issue completely changed the meaning of the treaty. In the Ethiopian version, Ethiopia remains an independent and fully sovereign country. In the Italian version, Ethiopia becomes an Italian protectorate, which is a major step on the road to becoming a full fledged Italian colony

Italian diplomats shopped their version of the treaty around European courts and most (other than the Ottomans and Russians) accepted the Italian reading. Emperor Menelik II only learned of the Italian reading of the treaty after he wrote letters to England's Queen Victoria and Germany's Kaiser Wilhelm, both who responded saying that they could not treat with Ethiopia due to the treaty - which came as a shock to Menelik. Menelik then publicly repudiated the treaty - which led to increasing tensions with Italy, and border clashes and skirmishes resumed. Eventually, Italy launched a full scale invasion of Ethiopia, which was shockingly defeated at the Battle of Adwa. In the aftermath of defeat, Italy signed a new treaty with Ethiopia, explicitly acknowledging Ethiopia's independence. The Battle of Adwa ensured that Ethiopia was one of the only African countries to survive European imperialism, and had significant effects not only in both Ethiopia and Italy, but also for Ethiopia's image around the world and in anti-colonialism globally

One final note - this example might be cheating your question a bit, as the consensus is that the translation "error" was not an error at all. Rather, the Italian diplomat who negotiated the treaty deliberately wrote the treaty that way and deceived the Ethiopian king, who could not read Italian, into signing what the Italians pretended was an identical treaty

Sources:

Rubenson, S. (1964). The Protectorate Paragraph of the Wichale Treaty. The Journal of African History, 5(2), 243-283. Retrieved June 18, 2021, from http://www.jstor.org/stable/179872

Use this link to get the full text of the Wuchale treaty

For more information about the Battle of Adwa and general Italian-Ethiopian relations at this time, I'd highly recommend this post on here from a year ago by /u/Georgy_K_Zhukov

The Grattan Fight (also known as the Grattan Massacre) is one such example with objectively disastrous results.

A group of Mormon pioneers travelling west on the Oregon Trail lost one of their cows and it was subsequently killed by a nearby member of a party of Lakota Indians. Nearby Army troops met with the leader of the Indians and were offered restitution, but the Mormon denied it (the Indians offered to pay by giving them a horse, the recipient wanted cash). The day ended with the matter unresolved.

The next day, a party of US Army soldiers based in Fort Laramie, which now lies in eastern Wyoming, was dispatched to arrest the Indian who had killed the cow (soldiers at Fort Laramie frequently met with the nearby Indians, and the Lakota had even signed a treaty there). They were led by a recent West Point grad, 2nd Lieutenant Lawrence Grattan. The young and inexperienced officer took a translator named Lucien Auguste with him. Auguste was not fluent in the Lakota language and, to top it off, became intoxicated during the journey to their camp.

According to observers from a nearby trading post, Grattan and his men approached the camp and Auguste failed to accurately translate the Lakota leader's words to Grattan. Grattan continuously demanded that the Lakota hand over the culprit to be arrested. Auguste then began taunting the Lakota, and they began to become agitated by the soldier's presence. Witness statements say an American soldier fired the first shot, which killed the Lakota Chief. His warriors then returned fire, and the entire American party was killed. Overall, there were 31 American soldiers killed, and the only Lakota casualty was their Chief, Matho Wayuhi (Conquering Bear).

This turn of events incited the Lakota to violence and led to what is called the First Sioux War, which really means that it kicked off decades of intermittent violence between the U.S. and the Lakota. This violence led to the deaths of thousands of Americans and Lakota, and the two did not end their fighting until the last Lakota band led by Chief Thatanka Iyotake (Sitting Bull) defeated the US Army at the Battle of the Rosebud and the Battle of Little Big Horn.

In short, a translator who wasn't very good at his job got drunk and pissed some people off, leading to over twenty years of intermittent warfare. Bad day at the office.

Sources: History of Wyoming, 2nd Edition, by TA Larson

The First Sioux War: The Grattan Fight and Blue Water Creek, 1854-1856, by Paul Norman Beck

Less of a translation error as such, but more of a conceptual translation error, and potentially it was a deliberate choice, but New Zealand’s Treaty of Waitangi/Te Tiriti o Waitangi.

The Treaty was relatively unique for its time, and remains unique today in terms of international law. At its simplest, and most coloniser centric perspective, it was an agreement between the British Crown and New Zealand Māori, signed 6th Feb 1840, that Māori ceded governance of New Zealand to the Crown and therefore became an official British colony.

The English version is fairly explicit on this stance. The Te Reo Māori version? Significantly different. I will try and summarise the differences below.

1. In the English text, Māori ceded the complete sovereignty of New Zealand to the Crown. In the Te Reo version, they ceded complete governance. This distinction in wording, as Māori didn’t have a concept of total sovereignty, meant that the chiefs who signed the Treaty believed they would be able to continue to manage their own affairs, with oversight and protection from the Crown.
2. The Treaty promised Māori the “full exclusive and undisturbed possession of the lands and Estates, forests fisheries and other properties which they may collectively or individually possess so long as it is their wish and desire to retain the same in their possession”. Like many pre colonisation First Peoples, Māori were in possession of swarths of land which they used for agriculture in a seasonal pattern. This meant however that a significant amount of their land would be left alone rejuvenating at any given time. The Crown, and the settlers, perceived that land as unpossessed, as it was not being actively used. So they took it, because the Treaty also gave the Crown exclusive right to buy land that Māori no longer wished to possess. In the Te Reo version, this was translated as “Te tino rangatiratanga”, or unqualified exercise of their chieftainship over all their lands, villages, property and treasures (taonga). It is unclear to this day if the Te Reo version and the Chiefs who signed it understood that the Crown was being granted exclusive right to buy, or if they believed it was first dibs.

It’s a very complex topic, and while the colonial government kept to the spirit, if not the letter of the Treaty for the first few years, by the 1850s they had more or less tossed it in favour of more underhanded land acquisition, and by the 1860s Māori were beginning to openly rebel against the Crown, leading to what is known as the New Zealand Land Wars in the 1860s. The repercussions of that are still felt for Māori today, and there is an entire government ministry dedicated to restitution for the historic dispossessions and traumas.

Further reading: Differences between the texts

The Waitangi Tribunal, including full texts of the treaties

One significant example is the Treaty Of Waitangi, the treaty signed between the British and the Maori residents of New Zealand in 1840, and which established New Zealand as a British territory.

The treaty existed in two versions: One in English, another in Maori. The discrepancies between the two versions have been causing headaches for the past 160 years.

The short version is the British version of the Text said the Maori would give up sovereignty of their lands, while the Maori version said they would give up governorship of their lands.

The issue was caused by the difficulty of translating the concept of "sovereignty" into Maori, at least for the people available at the time who could speak both languages.

The British view of the Treaty was, more or less, that by signing it, the Maori became British subjects, same as the Europeans, in exchange for completely giving up their broad rights to the land in general (not the land they occupied themselves, however) to the Crown. The Maori version was that they would become British subjects (same as the Europeans) in exchange for losing ultimate authority over the land in general but still being involved in the ownership and authority of the lands generally to some extent.

The second article of the Treaty says (in the English version) that the Maori give the Crown exclusive right to purchase their lands if being sold, while the Maori version says the Maori give the Crown the right to purchase their land if it is being sold. Even at the time, there seems to have been some disagreement on both sides, even internally, about whether this meant the Crown was the only entity who could purchase land from the Maori, or whether they just got first dibs.

From my understanding of the issue (and I'm not a Treaty scholar, just someone from NZ and interested in the British Empire), it's generally accepted there was no intention by the British to deliberately mislead or deceive the Maori chieftains with the treaty, and the issues were largely caused in translation by well-meaning people essentially trying to put together something on short notice and in a language not many Europeans could speak.

I am a bit late to this one, but giving my favorite example. In the Articles of Capitulation of Fort Necessity, Lieutenant Colonel George Washington of the Virginia Militia admitted to assassinating Joseph Coulon de Villiers de Jumonville, who was a French officer.

Background

In 1753 tensions were rising between the French and British in the Ohio River Valley, where both the British and French claimed much of the territory. In order to back up their claim, the French began constructing a series of forts. One of which was Fort Le Boeuf new modern day Erie, Pennsylvania. The British saw these forts as an attack on their claims to the Ohio River Valley, so Governor Dinwiddie of Virginia sent a young 21 year old Major George Washington to deliver a letter to the French at Fort Le Boeuf to leave. The French refused the request. Washington reported back to Dinwiddie who decided to build a British fort at the forks of the Ohio (modern day Pittsburgh, PA). Washington was to raise a regiment to command the new fort, but before the regiment was being raised, Dinwiddie sent a small force to begin construction of the fort immediately. The French noticed the British attempts to construct the fort so sent their own force to stop construction. Being greatly outnumbered, the British agreed and the French began construction of their own Fort Duquesne at the same site. Since the British were no longer able to construct a fort at the Forks of the Ohio, Dinwiddie instead sent Washington's force to defend against future French encroachments into the region.

Death of Jumonville and Fort Necessity

As Washington was advancing into the Ohio River Valley, he began to set up camp at a location called the Great Meadows. There he was joined by a small force of Mingo natives and heard reports that the French had sent a small scouting party their way. This ended up being a small group of about 30-40 men under the command of Jumonville who's orders were to determine if the British had entered French territory, and if so deliver a message that they should leave. Washington deployed his own party of mixed Virginia militia and Mingo natives to intercept the French.

The actual initial clash between the British and French has differing accounts depending on if you listen to Washington's or Jumonville's interpreter's account. What is known is that Washington decided to attack Jumonville who's men were quickly routed and at some point during the fighting, Jumonville was killed. After the attack, Washington returned to the Great Meadows and expecting French retaliation quickly built Fort Necessity, which was a small circular stockade.

The French indeed decided to retaliate and sent a large force to dislodge Washington from Fort Necessity. Since the fort located in a poor location and the British were outnumbered, Washington agreed to hear the French terms of surrender. The terms were sent in French to Washington who did not speak French so relied on his translator Jacob Van Braam. Washington agreed to the surrender and agreed to abandon Fort Necessity.

One of the terms in the surrender document was the admission that the British had assassinated Jumonville. However, Washington claimed that it had been translated to him as "loss of" or "death of" so was unaware that he was admitting to assassinating the French officer.

Aftermath

The actual translation issue where Washington unknowingly admitted to assassinating Jumonville is often attributed in oral tellings of the story to sparking the French and Indian War/North American Theater of the French and Indian War due to how the French reacted to one of their officer's being assassinated. However, the actual escalation of the situation was done by the British and not French so it likely did not play a significant role.

When the report of Washington's defeat made it to London, it was used to drum up support for a larger intervention against the French in North America and the British decided to send a large force under the command of Major General Braddock to dislodge the French. The British plans leaked to the French who dispatched their own force to Canada. Both forces led to the escalation of the conflict.

Since the escalation from small skirmishes to a full war were largely led by the British and French reactions to them, the mistranslation of surrender agreement likely did not have any major impact. So this is more of a cool story about the time George Washington admitted to assassinating someone.

Sources:

Young George Washington and the French and Indian War - McClung

Articles of Capitulation (of Fort Necessity)