Overall I have been pretty lucky. I have had guns that had problems, needed repairs or parts replacement, but very few that truly sucked.
From my broke college student years:
Keltec P11 - unreliable, picky about ammo, not accurate, marginal trigger, dropped magazines under recoil.
From when I was working at local gunstores in the '90s:
Norinco Browning Auto 22 Takedown copy - These looked decent outside but inside they were horrible, rough guns with soft steel. Everyone we sold came back and I think wefinally sold them of for $49.95 As-Is no return.
Llama psuedo 1911 style .380 pocket auto - This one was bought, sold, returned, repaired, resold, returned, repaired, resold, etc until the boss finally got fed up and shot it to pieces with a Mini 14.
From a old friend of mine:
Charter Arms Southpaw - Her father thought that it would be perfect for her because she is left handed. It clearly never had been test fired at the factory. The chambers had large rings in them from where the reamer had wobbled and the four of the chambers the cases had to be driven out with a brass punch and hammer. Wrote a chilly but polite letter to CA and send it back. When it came back it was so out of time that was shaving lead badly. Sent it back with a nasty, harsh letter that time on office letterhead and after several energetic phone calls they replaced it for her with a new gun. This one works fine and shoots ok, but she does not shoot it much.
Others:
Keltec p32 - the hammer spring broke in while it was sitting in a pocket holster in my pocket. They were entirely unconcerned and only vaguely apologetic when I call them about it. The did finally ship me a replacement spring, but refused to let me ship it back to get it checked out. i was so disgusted by it and it ended up sitting in my desk at work for about two years broken before I finally fixed it and traded it off.
Keltec Sub-2000 .40 S&W - inaccurate, unpleasant to shot, had a unnerving habit of popping open at the wrong times. Just felt like WAY too much power for that platform.
No, I am not a Keltec fan...