Wow, where to begin?
Rossi M-69. Every shot, the cylinder would unlock and try to swing free. That would prevent the subsequent trigger pull from indexing the next chamber. A firm slap on the left side of the cylinder would return the revolver to battery...for one more shot.
Colt 1911 (pre 1911A1). Would not feed anything but ball, and even ball was far from a certainty.
Springfield EMP. Wanted it so bad, bought one of the early ones (s/n 10xx). Beautiful, small, smooth. First box of 50 rounds had literally 20+ malfunctions. FTF, double feed,
many light hammer strikes, locking open on a mag with rounds remaining and failing to lock open on an empty mag. Nearly every type of malfunction possible. I suspect that the IL had something to do with the light strikes, but disassembly showed nothing conclusive.
Called Springfield, and they asked me to try a different brand of ammo. I did, and experienced more of the same. Sent them the pistol and the box of 50 empties so they could see the light hammer strikes for themselves.
Called back about a week later, and they had disassembled it and in so doing had broken something requiring frame replacement (?). So I had to wait until they ramped up on production again. About a month later, they sent me a replacement. Don't know whether to try it or sell it unfired...