Auto-Ordnance 1911 (pre-Kahr, early 90’s): My buddy and I bought identical pistols, save the finishes, at the same time (dealer claimed we were getting a special package deal). Neither would not get through a magazine without hanging up at least twice. Front sights fell off both to my recollection, but it didn't really matter as I would have had better luck hitting the target with a thrown rock. Local gunsmith said they were too crappy to work on.
Ended up selling mine to the same friend who owned the other problematic Auto-Ordance - so knew exactly what he was getting, an additional toy gun. He eventually (like 10 years later) sold them back to the same gun shop for about what we had paid for them, so I guess all was not lost. These two guns scared me off 1911’s for a decade, which is a shame.
CZ75B - (Aside: I like this gun). When I first bought it, premature slide lock every magazine our two. I had to re-shape the slidestop nub considerably with my dremel to get the problem to go away. It’s been fine several thousand rounds since.
Glock 20 (10mm):
Brand new gun, about every 5 rounds the magazine would fall out.
My buddies G20 was doing that a few months ago, so much that it was annoying me. I was accusing him of hitting the magazine release with the palm of his hand, and had him let me shoot it one-handed, with digits no where near mag release. Still dropped the magazine, ruining my know-it-all act. Both the magazine and the gun were low round-count, and the recess on the magazine for the catch also looked fine. But when he switched to a different magazine, for whatever reason, the problem went away.