That time the Austrian Army defeated itself
There has been no friendly fire incident in the history of the world like the 1788 Battle of Karansebes. The Austrian Army had been at war with the Ottoman Turks for more than a year when another contingent of Austrian soldiers stumbled upon another part of their army. What should have been a general misunderstanding turned into a full-on battle with more than ten thousand killed or wounded and the Ottoman capture of Karansebes anyway.
The story starts with a band of gypsies.
As every good story should.
It's necessary to know that the Austrians of this time weren't simply Austrian, they were fighting for the Hapsburg Empire, and their fighting force was comprised of several different languages, with no real common means of communicating between units. Still, units made up of these single language-speakers would regularly patrol by themselves, rather than joining other units to learn multiple languages or having a common tongue.
It was one of these units, a cavalry patrol, that was out looking for any signs of enemy Ottomans around. They didn't find any Turks, but what they found was a group of Romani Gypsies who were just settling in for the night. The Gypsies offered the Austrians a good time with dancing and drinking, which the grateful cavalrymen eagerly took. Then, more Austrians showed up, but these were a group of infantry, and the cavalrymen refused to share.
Anyone who's ever known infantrymen can probably guess what's about to happen.
This started a fistfight, of course. As the rival groups started fighting over the booze, shots rang out from across the nearby river. All the fighting Hapsburg men stopped fighting and took cover, quickly making it back to their camp to warn the others that Turks were shooting from the other side of the river. The camp exploded in a frenzy of men who thought Turks were overrunning their camp. When the German officers tried to get their fleeing men to calm down and come back, they shouted "halt," which in a German accent, was mistaken for "Allah." part of the Ottoman's battle cry.
All the sides fought one another until the camp commander believed he was being overrun, at which point he ordered the artillery to pound his own men.
Imagine this but with cannon fire landing everywhere around them.
When the Turkish Army did arrive to take the town two days later, it was completely deserted by the opposition. They rolled into the city immediately, and the Austrians didn't talk about Karansebes for another forty years.