Anco's basketball game (don't remember its name) was unplayable because of its bugs.
Lord of the Rings : at one time in the game (on the road to Rivendell), the dialogues start to be weird (weird letters, missing words...) and the game crashes or hangs if you speak to other characters. It turns back to normal at one point but you miss a (little) part of the game.
Legend of Faerghail : to bypass the protection, you have to give a name found in the manual (you have to look at the family tree of a character in the manual and give one of his ancestors name) ; problem is that no answer works. I found a cracked version some time ago and at last I can play the game ten years after having bought it...
Fate : Gates of Dawn had a bug too (it crashed or hanged at one point IIRC).
Bloodwych : Had one bug once that I could never reproduce. A fireball hit my party in a corridor near the beginning of the game (this fireball should never have been there and I couldn't find it again in subsequent plays). Three of my guys died, the other one gained about 15 levels at once and was almost invincible. Later in the game, at some points, the game hanged when you walked a certin way on certain squares (e.g. on one square it hanged if you turned left but not if you moved forward, etc.). I've started the game again later and could finish it with no bugs...
I guess I could find other ones as about 80% of the games I bought had bugs at one time or another
