What was the buggiest and or most unstable game you ever played?
I know Fallout 3, and Fallout: New Vegas (New Vegas especially are very prone to crashes and errors on PC). The one buggy game I remember would be Medal of Honor: Pacific Assault. It came out in 2004. Unlike The previous Medal of Honors EA used a different game engine, and wrote their own A.I. for the enemy soldiers. It had trouble running on my then less powerful computer back in 2004/2005. My computer just had a ATI Radeon 9200 SE with 128mb of memory.
However I do think it also had trouble running on more modern computers. The Japanese soldiers seemed to default to their banzai charge attack a little too often for my tastes. They ran out with rifles out and stabbed away with their bayonets. I did enjoy the medic system for healing and squad commands. On my old family computer it had some significant frame rate drops, and crashes to desktop. I think a patch resolved them later because I did beat the whole game on that computer! It touted having pixel shader 1.1 and Havock physics which was a big deal at the time!