Planescape , The Torment.
I played this superb great RPG for almost 4 weeks continuously and suddenly i find out that two weeks earlier i have mocked a character (the book keeper if someone is interested) which i shouldn't have done.
Because i had mocked him , he denied access to his house. Inside the house is a crucial mission to end the game.
Reloading the savegame would have taken me back for about 70 quests , so naturally , i was very very very very very very very extremely angry at the programmers to allow such a "bug". It has never been solved by the way.
I'm still angry ..