My old Kimber Montana 84M (stainless/synthetic). It left the factory with a laundry list of flaws so long I'd have to go back and look up my gunsmith bills to remember half of them. Firing pin protrusion was off by more than 1/3rd (too short), the crown was rolled in on itself, the headspace was so sloppy it made case-head-sep's like my 303 Brit ("match chamber" my foot!), etc. To make matters worse, the CGN'er who sold it to me swore it was a sub-MOA shooter, and the firing-pin protrusion problem which was about 2/3rds of the accuracy problem, caused occasionally inconsistent ignition, but not 100% of the time -- so I kept getting decent groups interspersed with bad ones, so I kept thinking that I was doing something wrong, rather than blaming the gun, and I kept working at it long past when I should have quit.
After $$$ at the gunsmith, and a lot of effort with handloads, I did eventually get it down to an honest 2MOA gun (down from 6+ MOA). At that point, I just wanted out, and so I sold it. The guy I sold it to (at a loss; I was a bit more forthcoming with the accuracy problem than the guy who sold it to me was) had Ron Smith re-bore the barrel in 338-08/338Fed, and it now shoots 3/4 inch groups.
Oh, and I almost forgot to mention -- it wouldn't feed shells on the left side of the mag. A mauser that doesn't feed -- go figure! It was apparently broken by design, as the replacement mag-box/follower that Kimber sent me duplicated the problem. I spent a few hours of trial and error with some dummy shells and a pair of pliers tweaking the mag-box, and I did, eventually get it to 100% reliability in feeding.... but ... wow...