I got stuck in Thief: The Dark Project in the Down in the Bonehoard mission for nearly a year, filled with continual reinstalls, mission restarts, etc. It was only once I turned up the brightness levels (which you're not supposed to do seeing as you won't be able to tell where the shadows are) that I could see the ladder behind a pillar that took me to the upper level.
Meh, I was 14 at the time. Probably too many drugs. That and no internet.
