I got annoyed the other day when trying to complete a random quest. Had to meet this orc character in a inn so I went over there only to have some homeless guy sneak up to where the orc was and start a fight. The orc started chasing down this bum outside of the inn so I followed. The guards went after the orc however and killed him, ending the quest. Didn't save for a while before that so said screw it and then later tryed rezzing him to find out that it didn't matter, the quest wouldn't complete no matter what.
I did manage to fix the other "vow that breaks" quest that was broken for me.