I once asked a B-17 pilot if planes in a bomber formation didn't often shoot their own fellow machines down in combat. His reply was that it happened all the time. Although the bombers flew in formations designed to minimize fratricide ,as well as provide overlapping and self-supporting fire to their fellows, in the heat and confusion of action, when men were fighting desperately to stay alive, accidents happened.

Gunners shot their own comrades (mechanical stops prevented them from hitting their plane itself), mid-air collisions happened, and sometimes even bombs were dropped on squadron mates. I've often wondered just what percentage of combat casualties are caused by friendly fire. I imagine it's a lot higher than the military would like to admit, if it even knows at all. Still, given the nature of the activity, it's certainly not surprising and to a certain extent, unavoidable. Training and proper procedures, good communication and command may help reduce the problem, but I don't see how it can be entirely avoided.

When I was in the Navy, the helicopter on my frigate shot itself down during a search and rescue training mission. The stops on the door guns weren't properly positioned, and a round nicked a rotor tip, unbalancing it, sending the helo into the water. Fortunately, we rescued the crew quickly and there were no injuries except for a broken arm and some badly bruised egos.