A friend told me about this (possibly apocryphal) Battle of Karánsebes, a 1788 battle during the Austro-Turk War in which the Austrians attacked their own troops not only once, but twice within about an hour; these two fights cost them 10,000 men out of the 100,000 in their army, and allowed the Turks later to easily take the city of Karánsebes (today spelt Caransebeş, in what is now Romania).
This story is ridiculous, and also potentially untrue. The only sources for this battle were written many decades after the fact.
Basically, here's what happened:
The whole damn Austrian army was getting read for a huge fight with the Turks. The Austrians camped out around Karánsebes, obviously split up their army into smaller camps and groups, and sent some of those groups out to scout for Turks. One of those groups was the army's hussars: light, fast mounted soldiers, who also dressed like total pimps and looked like this (roughly):
Their uniforms have nothing to do with the story. They're just cool.
Okay, so, the hussars went out scouting and found precisely no Turks. What they DID find, however, was booze, because OF COURSE THE HUSSARS WERE THE ONES TO FIND THE BOOZE. They ran into a group Romani and bought a butt-load of schnapps (that's a metric butt-load, by the way, because they're European) and proceeded to get drunk-drunkity-drunk-drunk.
Then some of the Austrian infantry (foot soldiers) caught up with them and were like, "PARTAAAAY!" But the hussars went, "Nope." And the infantry were like, "Guys, we do all of the horrible, horrible fighting, face-to-face, on foot. We deserve a booze or five. Give us a schnapps."
And the hussars went, "We don't partaaay with the likes of you. Have you seen our jaunty jackets?"
And the infantry went, "Srsly, share."
And the hussars went, "No. Shan't."
And the infantry went, "Right-o. I guess we'll be fighting, shall we?"
And the hussars went, "Oh, I should think so."
The hussars decided to REALLY protect their booze, so they actually took barrels and carts and horses and drunkenly built fortifications around their schnapps. The infantry then decided to besiege their fortifications. A scuffle ensued. Somewhere inside the scuffle, someone (from which side is unclear) fired a shot. This immediately escalated the battle from a squabble to full-blown combat.
This is "The Austrians Attack Themselves #1".
In the midst of them killing each other, some of the infantry (for some reason, I have NO idea why) started to shout, "THE TURKS! THE TURKS!" This made both the hussars and the rest of the infantry think they were about to be surprised by the Ottoman army, so everyone promptly fled (and presumably left the schnapps unguarded).
Now, keep in mind that the Austrian empire covered HUGE territory and its army comprised not just Austrians, but also Italian regiments, Balkan Slavs, and many other ethnic and linguistic groups that couldn't understand each other or their leaders beyond a few simple commands. So when some of the leaders at this drunken skirmish realized that they weren't being attacked by Turks, and tried to yell "Halt!" ("Anhalten" in German), many soldiers who did not speak German misheard this as "Allah!" and were confirmed in their fears that the Turks had arrived.
Now there were two huge regiments running and screaming through all of the Austrian camps in the middle of the night, saying that they were under immediate attack from the Turks. One of the corps commanders at the camps, who could see practically nothing in the dark apart from a huge surge of mounted troops barreling at them (that would, of course, be their own mounted hussars attempting to flee back to camp), though that the hussars were Ottoman cavalry bearing down on the camp while they slept.
He ordered artillery fire and shot his own troops.
This would be "The Austrians Attack Themselves #2".
There was such a blind panic that the entire 100,000-person army actually packed up and retreated from the imaginary enemy, occasionally firing on itself. This killed one out of every ten men–and gives us a literal example of the word 'decimation' (which actually means: "killing one out of every ten", not just "suffering huge losses" like many people think it means).
Two days later, the Ottoman army actually DID arrive. They found heaps of wounded and dead soldiers, but no actual army to speak of. So they took Karánsebes. The End.
(For a brief summary of the whole war, not just this one incident, see this).