Every once in a while I get a flap shot off in when in lowered position. The only problem is the flap is clearly gone and listed as damaged, but the lift on the wing is the same as when the flap was shot off (in the same lowered position). The extra wing lift from the flap doesn't seem to change even when changing flaps to full up or down. This isn't a big deal, but figured it could be considered a bug.
I, however, have used this to my advantage, like continuing a turning dogfight and landing. The only disadvantage is in level flight, having one flap (the missing one) notched down obviously reduces the max speed of a plane. I have experienced this mostly on the P-51's. I don't remember the other planes, but I do remember it happening in non-P-51’s.