Though not exactly "Friendly Fire" I remember reading about an incident where Swiss ME-109s were escorting a US bomber to be interned when a US P-51 pilot noticed what was happening. He thought the bomber was being attacked by Germans and went in and shot down one of the 109s.

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For the first time I have seen "History" at close quarters,and I know that its actual process is very different from what is presented to Posterity. - WWI General Max Hoffman