Cetme Sporter... 20rd mags wouldn't lock in without filing. Would not fire, headspacing issues I'd guess.
Jaeger .22LR M16 clone - broke firing pin. New one custom made.
Mossberg 500A - super old, put together from random parts, both extractors on bolt were right-side hook type, would not eject shells/casings; I dremel'd off the hook to a 45 degree angle.
DPMS Classic 16 - brand new, factory gun; even with iron sights maxed out, would not adjust enough to hit POA. Back to factory, loose barrel nut or extension. Gun sounded like scraping stone when charging.
Oly Centurion 15 - slam fired badly. Traded back in.
CAI AK74 sporter - rivet fell out from front trunion.
Waffen Werks AK74 - Came finished with GunKote that would scrap off like a rubber sealent. Sent off for duracoat.
Saiga 12 - was unconverted. To fix: converted it.
Charles Daly 12ga - spent casings hung in the ejection port, traded in.
Mossberg 590 - damaged mag tube caused follower to stick after X amount loaded in. Safety stuck bad until broke in half, missing bearing. Replaced mag tub and alum safety switch.
PSL54C - bolt would hold open when firing. Magazine springs were backwards inside.
Rem 700 SPS Tactical - Xmark Pro trigger doesn't really adjust for break weight whatsoever.
FEG+TGI AMD65 - won't fit new commercial Bulgy poly mags. Finish is a crap paint. Folding stock release button is very hard to depress. Tapco galil-type handguard was custom fitted by me, but loose as hell.
AR15 build - Mostly Seekins parts. Rails are too big to accept slide on and bolt type accesories (Nikon M223 scope mount for one).
AR15 SPR type build - Wilson Combat barrel and YHM gas block and upper. overgassed like crazy. Installed Seekins adj block
I'm sure there's more.