In Resident Evil [the first one for the gamecube] if you beat the game under a certain time limit, you can unlock a mode that turns all the zombies invisible. You don't see them until they touch you.
Jet Set Radio Future had this graffiti soul unlock system, so in order to beat the game completely you had to get all the souls. I remember there was a few that took me forever to get. One was in Shibuya Terminal where you had to jump onto platforms without jumping onto the same one twice [or falling off said platforms]. Because of how my version of the game [the one with sega gt in it] was made you had to do a little extra work in order for the task to be completed.
The other task was in the sewers. There is a huge tower off to the right of the stage and you had to wall grind the inside all the way to the top of the tower. It was easier said than done. Between this task and Shibuya's I think I clocked in at least 4 hours of work there.