Societies are not measured against one another based on their advancement through some linear Civilization-esque technology/innovation tree.

Foragers are not stupid compared to agriculturalists. One society is not better than another because they invented the wheel, or metallurgy, or writing. Avoid assumptions of superiority when examining the past (and the present, for that matter) and look at the specifics of how each culture worked.

Presentism, many people don't understand how people could willingly stood in a line and fired inaccurate and semi-deadly weapons. They look at present military ideas and think that they could arm chair general victory despite not fully understanding the technological limitations of the Early Modern Era.

Sailing is a real skill, not something that's done with a horde of grumbly half-drunk men who only comply because they fear corporal punishment.

Biggest problem: Confirmation bias.

People tune out anything that doesn't reaffirm what they believe, and try to find what reinforces their own notions rather than try and challenge them. Confirmation bias is a huge problem in the topic of Israel.

In the other topic, people should know how vague international law and theory really is would be great. A lot of people think something is clear-cut when it's really, really not.

Roman politics were a lot more complicated than just about anyone understands. It's really true that the inner complexities of Roman politics are extremely difficult to understand, and there are only a handful of people who understand the whole picture. It takes years and years of extremely intensive and annoying study to be able to talk about Roman politics on an academic level, and reducing it to simple terms is extremely difficult--you're bound to leave something out, and that something is bound to actually have some significant importance at some part of the proceedings.

The difference between behaviour and identity.

Our current understanding of gender and sexuality is incredibly reliant upon identity, so it's very hard for laypeople as well as historians to see sexual practices as (potentially) divorced from identity.

How little we know about the ancient world due to how little evidence survives. This recently summarised by Robert Parker with:

The books about Greek and Roman religion do set it down as though we know 95% of the picture when perhaps we only know 20%.

I wish people better understood how diverse America was at the time of the American Revolution, and how many sides there were to the conflict. It wasn't American vs English, or even Americans and their Indian allies vs Britain and their Indian allies.

Over one third of those who took up arms for the American side and fought for the "rights of Englishmen" weren't English or descended from Englishmen. Germans, Irish, and Scotch-Irish comprised some 40-50% of enlistments in the American army in the middle colonies.

There were so many Germans fighting in the Revolutionary War that there were several battalions formed of German speaking soldiers, led by German speaking officers.

On the other side of the conflict, a large portion of the British Army was Scotch or Irish. In fact at one point during the conflict a group of recruits who were recruited for a Highland regiment, mutinied when they were told that they were being assigned to a different regiment. Why? Because, according to them, they didn't speak the English language, they didn't dress the English way, and they didn't have English customs.

Almost every battle in the war featured Americans fighting against other Americans--many battles featured Loyalist militia vs Whig militia.

The other important concept that I wish was better understood was what line warfare was about, particularly when it came to campaigning in North America, where the terrain made large formations (such as were used in the Seven Years' War in Europe) impossible.

Presentism

As /u/DonaldFDraper points out, this is a pretty big problem. This is especially the case in the Spanish Civil War. As Jose Sanchez said, "The Spanish Civil War was one of the great mythical wars of modern times.  People everywhere, and especially abroad, saw what they wanted to see." This was true while the war was going on and it is also often true today. Due to the fact that there were a myriad of groups that made up the combatants in the Spanish Civil War and nearly every political idea had its adherents during the conflict, it can be very tempting to find a faction in the Spanish Civil War that you like or dislike and proclaim that the Spanish Civil War is just like what's going on right now. This ignores the complexity of the war and just how tied these issues were to 1930's Spain. Still, it's all too common to find people using the Spanish Civil War to advance a current political agenda by uncritically applying the past to the present (or applying the present to the past for that matter.

Nuance.

The subject of my flair here deals with papal history. The Catholic Church in general and the papacy in particular are complex topics (and I say that as a Catholic myself). Few issues are able to be defined simply, and it is very easy to miss the nuance present in papal statements and actions. Recognizing trends in papal thought often comes down to recognizing tone, particularly tone relative to previous statements on the subject. Months ago, I wrote about the concepts of "Papalese" and "Vaticanese" as languages that are analogous to the idea that diplomatic language can't be read purely on a literal level. I think that a lot of misconceptions about papal statements come about because people miss the nuances present in those writings.

Lockhart's notion of Double Mistaken Identity, most straightforwardly summarized and applied by Mathew Restall in Seven Myths of the Spanish Conquest. In encounters between different cultures, it's unlikely that simply not understanding each other's languages will be a factor, or differing technology, or whatever.

Rather, when you look at an encounter such as the interactions between Moctezuma and Atahualpa with the Spaniards, it's important to realize that each side is assessing both their actions and those of the other side according to their own values. So when you have the bulk of the history written by one side, and coincidentally their version makes the other side look foolish or savage or crazy or superstitious, it's possible that in addition to propaganda, those telling the story are simply trying to fill in the blanks as best they can, without having any real sense or empathy for the actual decisionmaking criteria the other side had in play.

Number 2 would probably be what I call the Builder Fallacy, namely that a culture was at its peak when they built the most still-extant big structures. I guess this could go with the above reservation about Civilization-like linear progress, but I think it's a bit narrower.

Not a professional historian, but there is something about the history of sexuality, and history of Christianity, that gets incredibly misunderstood.

Basically we tend to consider that sexually "open" or "progressive" or "tolerant" periods (Pagan Greece / Rome) were good and "repressive" or "prudish" periods (Christianity) bad. Why would anyone try to repress harmless fun? So bad/stupid right?

But the problem is, that we forget that a huge amount of the sexuality that was repressed was rape. Which puts things entirely in a different perspective. For every couple whose voluntary, consensual harmless fun was repressed, multiple cases of selling daugthers, wives, war captives etc. to sexual slavery, to the rape industry (pornai) were repressed.

Given that rape was so widespread, this should color our views every time we read some old, "prudish" text about how sinful is fornication, how sinful are the desires of the body etc. etc. a lot of what they understood under fornication was rape, or at least had some kind of element of force. For example in Corinthians (I think the first epistle) Paul condemns a man who was expected to financially support the young widow of his deceased father, and figured he might as well abuse this situation to blackmail sexual services out of her. This and things like this were a huge part on why Christianity was so rigid and prudish about sexuality and considered the desires of the body so sinful.

Think that sex is nothing but harmless fun and therefore does not deserve repression implies mutual consent and if you assume that the majority of sexual acts in the "open minded" Ancient Greece or Rome were based on mutual consent, you are very, very mistaken.

It is fairly well demonstrable that every time the situation was more consensual, there was less repression and less prudishness. For example just living with your long term partner, unmarried, and having sex, is called clandestine marriage, and actually English society accepted it up to 1753.

The idea that international law operates in the same way as law within a state

That in the terms of the Vietnam War, there was no systematic policy of genocide carried out by the US Government against the Vietnamese people. This is something that is repeated over and over again by laymen who believe they know the "truth" about the war (usually alongside 'false flag' and what not) as well as by authors writing for a broad audience ('Kill Everything That Moves' is the best example of that).

The issue of war atrocities during the Vietnam War should be put into a broader context and analyzed out of different factors involved and what actually happened on the ground. What no one can deny is that they happened, which seems to be the opposite side of this coin, and we have plenty of proof to see that this happened more than a few times.

My beef: second opinion bias. Learning that what you've been taught at school was an oversimplification doesn't mean that the information is inherently wrong. It just means you should learn more about the subject at hand! Especially in K-12 schools, there's a limited amount of time to go over vast amounts of information. Because of this, information is going to be glossed over, oversimplified, or otherwise not mentioned. This doesn't mean your teacher was 100% wrong on everything just because they didn't take the time to go through the nuances. It means read more! Going the other direction and proclaiming everything you learned at school to be propaganda or total lies is going to impede on your understanding of events, and that's not good.

Also I would love it if people can understand basic statistics and how it works. Demographers and statisticians do not just pull shit out of their asses. Large sample sizes do not inherently mean good and reliable. Etc.

I'm only a student with a minor in History.

My God, the amount of ordinary folks who actually know anything accurate about the USSR or Russia is neglible.

A lot of people don't seem to realize that wars aren't neat affairs. They're won by killing as many of the enemy as possible while minimizing your losses, because it's the easiest way to end the war. Every action in a war is to this goal. This came up recently, but the atomic bombing of Nagasaki and Hiroshima are perfect examples. People said it was barbaric, and a war crime to directly target citizens. But you have to look at it from Washington's point of view. The alternative was to launch an invasion of Japan, causing massive losses to both sides. By bombing those two cities, we forced the Japanese to surrender by making it quite clear we had the ultimate weapon, and weren't afraid to use it. That actually saved lives, because it prevented a very bloody invasion.

Tl;dr War is hell