My first was a Gen 2 Keltec p3at. It was a total POS. It FTE and then double fed once per magazine. It also stovepiped and cracked the magazines at the front from the feed ramp hitting the mag. It went back to KT and returned a few weeks later with a hand written note saying it was test fired with Rem UMC FMJ and it cycled perfectly. I tried it with that ammo and it still jammed.
I traded it to a local shop after telling them it had issues. They gave me $100 credit for it towards the purchase of a Smith and Wesson 442 and haven't looked back.
My second was a Remington 870 Express 12g. I bought it with the 28" vent rib barrel and picked up a 18" Remington HD barrel. The shell latch would not release a shell from the magazine about 1 in 4 times the action was cycled. The HD barrel would stick shells in the chamber so bad that the extractor would rip through the rim of the shell leaving the hull in the chamber.
I called Remington and they told me the problem was I was using crappy ammo. I told them I was using Remington shotshells. Turns out Remington does not service their own weapons. They gave me the address to a gunshop in B'ham, AL and told me to ship it there for service. 4 months later I got it back with a note saying the shell latches were replaced and the chamber of the barrel was polished.
The shells released from the magazine now, but still stuck in the chamber. I immediately sent the barrel back to the same gunshop and gave them a phone call. About a month later I got a new barrel in the mail. They replaced the Express barrel with a 870P barrel and the gun worked, but I had no confidence in it.
I traded it in for a 870P with a wood stock. I kept the 28" vent rib barrel to use for hunting.
Buying a lemon sucks, but if you buy and trade enough it will eventually happen.