The Niš incident

 (November 7, 1944) (Yugoslavia)
(In the land that is currently known as Serbia, as reports state)  

A squadron of American fighters (P-38 Lightnings) encountered a German train and decide to attack it, but face a few losses as the train is defended with artillery guns.
Enraged at losing a comrade, the Americans later spot a moving group of tanks and military vehicles in a convoy towards the nearest town so they decide to get their revenge by attacking them because they assumed they were German.

Unfortunately, they weren't.

The convoy was a parade of allied Soviet and Yugoslav partisan troops moving together for symbolic purposes and were surprised to see their allies attack them and some of them even tried to wave their flags higher for the planes to see, but to no avail. 

Then, the Americans spotted an airbase which they assumed incorrectly again, was a German base but it wasn't. 
This led to the base being attacked and it's parked fighter planes being hit by incoming gunfire, and sadly many lives lost from both sides as there was also a huge dogfight with defending Soviet planes against the American planes. 

Eventually, when reinforcement Soviet planes were sent in, one of the Soviet pilots tried tilting the wings to reveal the red Soviet star to their allies, to signify they were friendlies. 
It didn't work at first but, thankfully, one of the American pilots spotted the red star and ordered the other planes to cease fire immediately and left the site.

Meanwhile, the American pilots of the downed planes who survived were either handed over to British intelligence or crossed to allied lines from Yugoslavia, after being captured by them.