Reading Tom Given's newsletter -another thread, he describes a malfunction that couldn't cleared easily. The slide was forward of where it should have been. Thus, the user was out of luck without a BUG.
That got me thinking of similar non-fixables that weird (not just a broken spring, have had that).
I go for two, I had:
1. A 2nd Gen G19, the empty casing was impaled on the firing pin that was sticking out. Like a stuck pig through the primer. It would not flick off.
2. Another 2nd Gen G19. The empty casing rotated in space such that the mouth of the casing was facing backwards towards the breech face and horizontal. The slide slammed on it and crumpled it a tad into the face. That was a total stop and get out the anvil to heat the gun up and hammer it (just kidding on the last part).
Others?