Battle of Karansebes was fought as part of the Austro-Turkish wars. The Austrian army, stationed in anticipation of the Turkish attack, was made up of soldiers of different linguistic backgrounds. Night before the battle, some Austrian soldiers sneaked in alcohol and overtime everyone became drunk. Not understanding each other's language, the drunk Austrian soldiers had a fight which turned into a mass brawl and eventually 10,000 Austrian soldiers died in the ensuing violence.
This makes it the worst case of friendly fire in history. When Turkish soldiers came the following day, expecting a heavy fight, they found everyone was dead already and took the city without doing anything.
https://en.m.wikipedia.org/wiki/Battle_of_Karánsebes