During the civil war in Rome, between Octavius and Antony, early in the war, there was such an instance.
Keep in mind this was a civil war, and at this point it was no longer about preserving the republic, or vengeance for the death of Caesar. This was simply about who would run the empire. And these armies had once been allies, all fighting against the forces of the senate, who had killed Caesar. Many of the men, especially the officers and centurions knew each other.
As the armies were camped, preparing for battle some of these soldiers crossed over into the opposing camps to talk to each other. The end result was officers from both sides telling their respective leaders that they should really try to work something out, as nobody wanted this battle.
And as far as the average soldier and even officer went, there was nothing much to gained by it. There was no booty to be had, no vengeance, no real honor, other than that of simply defeating their foe.
These were fellow Romans they were fighting, and in many cases friends and family who didn't even have any real political differences.
A truce was made but, of course it didn't last.
Well, there was the Christmas Day Armistice of 1914, where during WWI, the British and German troops agreed to a ceasefire on Christmas Day, and stopped fighting, even going as far as to have a friendly soccer match. However, a few days later, the fighting picked up again, and didn't end until 1918.
The documentary series "The First World War" (I'll dig up the specific episode and link it), mentions the live and let live attitude in the Western Front between the Triple Entente and the Germans.
Basically both sides were tired of fighting, and thus would avoid shooting at the enemy when they were exposed. There's even an instance where a rock landed in a British trench with a note attached that said something to the effect of: "we've been ordered to launch an artillery strike tonight at x time, but we don't want to, so we'll whistle when the attack is coming." The Germans were true to their word and the British got out of the way, with no casualties.
All the episodes are on youtube, but I'm going to find the specific episode they mention it and link it here to make things easier. Plus it's a fantastic documentary.
EDIT: Ok, I found it. It's episode six: Breaking the Deadlock. Here's a link to the video, it'll start you off at around 16:30 and the relevant section ends at around 22 minutes: http://youtu.be/GVrULXaxWz0?t=16m24s
There were several battles where armies of mercenaries refused to fight each other in the 11th century, under the Buyids, in modern day Iran/Iraq. Basically you had lots of petty lords who would hire mercenary armies/buy and build slave armies, and it was in the interest of the mercenaries to keep the wars going as long as possible so they would refuse to fight decisive battles (usually their master wasn't there when they did!).
There are several instances of this happening, detailed at quite some length in Roy Mottahedeh's "Loyalty and Leadership in an Early Islamic Society," which is quite good as well. If you want specific examples, I can get you some when I get home and get the book.
I would encourage you to check out the documentary "Sir No Sir" about GI resistance during the Vietnam War. The US gave up on ground troops and infantry in the war, switching over to an almost entirely airborne campaign in part at least because of the fact that they really couldn't trust their soldiers to fight. You also had widespread "fragging" of officers: enlisted soldiers assassinating their commanding officers. This goes beyond a battle, and the opposing side--the Viet Cong and North Vietnamese Army--were more than happy to keep the contest going, but a significant chunk of US soldiers were radicalized to the point of sympathizing with the enemy.
This is also at least a significant part of why we got rid of the draft and transitioned to an all-volunteer military. Not only was the quality of soldier generally much less in a conscripted army, the potential for sabotage was much much higher.
After the eruption of the massive gunpowder reserves stored on the L'Orient during the battle of Abu Kier Bay (also known as the Battle of the Nile), it's said that both sides were left in such shock at the massive and sudden loss of life that nobody fired for 10 minutes and Nelson had boats sent to save what few survivors there may have been from the enemy ship.
Source: William James's "The Naval History of Great Britain, Vol 2" (1827)
The English Civil War was known for this phenomenon. Infantry in this war were largely pikemen and sought to win a shoving match known as 'the push of pike' where both pike formations would attempt to roll over the other. Now this sounds pretty messy, and it's hard to imagine how such combat didn't turn everyone involved into Swiss cheese. In reality casualty figures are quite low and many pike engagements were entirely bloodless.
Many infantry weren't too keen on getting run through so as often as not they would march at each other, get within like waggling range and then just stand there waggling their pikes at each other from a safe distance until one side or another withdrew. They wanted to make enough show of fighting to convince their officers without getting into actual striking distance. What you have are hundreds of men waving sticks at each other and shouting while occasionally looking back to make sure their commander isn't getting suspicious.
Check out Mike Duncan's 'Revolutions' podcast for a great breakdown of English Civil War tactics.
The Medes and Lydians fought a war in 585 BC that apparently stopped in mid-melee because of a solar eclipse. According to Herodotus (The Histories, 1.73-74), the battle was ongoing without one or the other side gaining the upper hand when suddenly "day changed into night" due to the eclipse. The soldiers at that point refused to continue, stopped fighting, and agreed to terms.
We know that there was a solar eclipse on May 28th, 585 BC, so this pretty much dates the battle. Of course, Herodotus often got his dates mixed up and was notably sloppy about rather a lot of things, so the alternative hypothesis is that the battle took place during a lunar (not a solar) eclipse, and so it was actually in 609 BC. Ah well, whats a few years here and there in the cosmic scheme of things :-)
tl; dr: nothing like an eclipse to stop folks from fighting.
At Wallingford Castle in England in 1153, the barons leading two opposing forces refused to fight one-another. The Empress Matilda and her cousin King Stephen had been fighting for the throne in a 15-year civil war, which ruined much of England. Law and order had broken down, and the period became known as The Anarchy.
Both Matilda and Stephen's barons were sick of the constant turmoil, over a decade of fighting with no clear victor, and the lack of an effective monarch. Furthermore many barons on Stephen's side saw Matilda's son Henry as the rightful heir to the throne, and the church was also keen on a peace.
Henry and Stephen signed the Treaty of Wallingford, and fighting in England petered out soon afterwards.
There was the standing convention that Swiss mercenary regiments on opposing sides would not fight each other which resulted in a number of occasions where the two sides stopped while the battles were raging around them. Given that medieval and early modern battles were set-piece engagements, this was mostly due to thoughtless deployment, as the Swiss mercenaries needed to be re-deployed in such a manner that they fought against non-Swiss opponents.
An infamous breach of the convention occurred during the battle of Malplaquet when the Swiss regiments May (one in French, the other in Dutch service) clashed during a surprise encounter in the battle, see Swiss Historical Dictionary, French text.
Ignorance of this convention still confuses many modern English speaking historians and leads to inaccurate analysis such as at the battle of Baylen 1808.
The Border Reivers who were the families who lived on the English/Scottish border in the 13th-17th century were raiders and mercenaries. The families tended to be interrelated and the ties to the English and Scottish crowns were not too strong due to the distance from the capitals and general lawlessness near the border. There are several reports of troops from the borders meeting each other in battle and putting on the appearance of fighting whilst actually talking to each other
Source: The Steel Bonnets: Story f the Anglo-Scottish Border Reivers by George MacDonald Fraser
It’s not a battle and hence not directly applicable, but there are certainly instances of American soldiers refusing to fight in smaller-scale confrontations, for instance along picket lines. In Bruce Catton’s Civil War, the author describes how the soldiers would arrange to swap tobacco for whiskey, for instance, building a small boat and loading on it a few bottles to be sent to the other side. The boat would return in due time with the enemy’s goodies on top. There’s supposedly another incident in which a bunch of Union soldiers were actually invited beyond the Confederate lines to hold a dance, and great fun was had until a Confederate officer happened by. Being an officer, he of course determined to take the whole gang of Union soldiers prisoner. But the distraught Confederates intervened, pleading with the officer that they’d given the Northerners their word of honor that they’d be safe, etc. The officer, upon hearing this, decided on mercy, gave all parties a don’t-let-this-happen-again lecture, and the Union soldiers skedaddled.
(Bruce Catton is known more for capturing the spirit of the time than for his academic rigor (although he read a hell of a lot of regimental histories, from which stories like these presumably come), so take the above anecdotes at arm’s length.)
The senate would send armies to stop Sulla. The senate armies would break camp and defect when ordered to attack Sulla.