Assassin's Creed Tail Missions
Undoubtedly the most infuriating problem with all of the AC titles is the ridiculous tail missions. The ones where you have to follow important character x for half an hour, listening in on conversations. Knock anything over or fail to duck into cover quick enough and bang - you're right back at the start.
Worse, they added a mission rating system in in ACIV - everyone I know who played rated the tail missions one star out of five, they seem to have ignored it though. I might pick this up in a bargain bin - but as a day one purchase? No way.