The "funniest" one I've ever heard about is Battle of Karansebes, where an army was defeated without the enemy even being anywhere near them.
In a nutshell, the story is that the Austrian army (100,000 soldiers) were camped somewhere. During the night there was an argument between some drunk soldiers and one of them started shooting. Then mass panic broke out and some soldiers started shouting the enemy (the Turks) had arrived. Many soldiers immediately fled the camp and others just started shooting at anything that moved. 10,000 of their soldiers died that night and two days later, the Ottoman army really arrived and easily conquered the city.
Apparently historicans are unsure whether this is really what happened, but it's a funny story nonetheless.