A few of the games mentionned here I played when patched and thus didn't have many troubles with them. But the one that gave me troubles when I bought it was The Temple of Elemental Evil. I remember it kept crashing when I tried to go into some parts of the elemental nodes, and I only completed it when I started a new game with the Co8 patch.

On my old PC I would get to Hollywood and it would just restart my PC. I did EVERYTHING I could because I totally sunk into this game, I loved it.
It wasn't until I got my recent gaming PC that it actually started working (using the unofficial patches of course)

Not really a bug but I bought Grand theft Auto IV for PC the day it released and Man I did not realize what I had got myself into, first of all the performance is crap for example on lowest settings i only get around 30 fps and I have a fairly decent computer too plus the online was full of modding freaks that would make there SMG's shoot RPG's and stuff similar to that. It's just an over all bad port for a game that could have been perfect.

Unplayable? I've finished it probably 5-6 times with the retail version and 1.2 patch before I even started using the community patches
Probably the most consistently buggy game I've ever played was Operation Flashpoint. Amazing game but buggier than a very buggy thing thats got extra bugs. The issue was as much exacerbated by patches as it was helped by them. I used to help run the official forums (before codemasters decided they did want to run their own forum) and each time a patch came out, we'd joke that we had to get it to find out what new and exciting bugs BIS has made. I swear the mission Pathfinder never worked the same twice. In a way it was a good thing, you always got something unexpected but that is a lemons to lemonade viewpoint

Arx Fatalis would crash to the desktop about every 20 minutes or so. I became a compulsive quick-saver before the end of the first dungeon in that game....

I said almost unplayable. It is very possible to progress through the game and not encounter one of the game breaking bugs, but for most people it is far more common to run into them multiple times, thus preventing you from actually finishing the game. For me, it was always the infamous "Society of Leopold" crash, which was supposedly fixed by the official patch, but still seems to happen quite a bit. It wasn't until Wesp and his unofficial patches came along that particular bug finally got out of my way.
Had I experienced these kinds of problems with any other game, I probably would have given up on it long before discovering the unofficial patches. But, since VTMB is so very good, I stuck with it and still occasionally play it today.

Neverwinter Nights 2 at launch. God it was a train wreck. I finally gave up on it after about 1-2 weeks. I came back to it something like 6-8 months later and had a ton of fun with it.

As I mentioned in another thread, I couldn't play Undying at all, because it locked my computer or rebooted it constantly. I was rarely able to play for more than five minutes straight without my computer crashing.
I never had any significant problems with Bloodlines in my 3-4 playthroughs. The only noticeable bug I ever experienced was that debris would stick to me if I walked through it while crouching.

I never encountered (though I've heard about it) the Society of Leopold crash, and I played the game unpatched. It's funny, people mention that VTMB was incredibly buggy, but aside from no audio (because of an under specced machine) I didn't run into any...
Maybe I just got lucky? Or maybe I wasn't looking hard enough. Whatever the case.

You know, I was having a really weird graphical glitch with NWN2. When my torch was on fire, it produced no light. When I doused the flame, the room lit up nicely.

Before I got a newer, higher-performance computer, I remember one glitch that tended to happen often in high-graphics games: the floor would vanish. I would look down and see that I was no longer standing on anything. It might not have any effect on the gameplay, but it was still weird.
In Morrowind, I found that I could sometimes walk through walls in building or fall through staircases, and then plummet through an endless grey void. Eventually I'd land back in the map, somehow, but it was a surreal experience nonetheless...

Damn! I almost forget to mention the real winner: The Temple of Elemental Evil. The story is pretty good, the combat system is great, the characters are cool and the graphics are beutiful... but it's almost unplayable without the unnofficial patch.

I had a non-GOG version of Gothic 2 whose bugs would DECREASE as you went up in levels. I think it was a World of Gothic version or something. Once I installed Night of the Raven the bugs became worse to the point where I could not play the game anymore. The bugs were of the usual CTD variety.

OFP was another of those ones where increasing the resolution actually helped the smoothness, I think that was just evil magic