In my AP US History class last year one kid somehow found the exact test that was being used for the final online. He missed just one answer. I'm not sure if that was just a mistake or some sort of genius, but our teacher thought he was a savant. Pretty much everyone in the class knew he did it, but nobody told. He was a pretty big jerk, and I definitely considered telling, but in the end I figured that I did really well just with my own effort, and if he can somehow deal with being a cheater after spending a year working his ass off (or maybe not, seeing as he was willing to cheat on the final), then more power to him I guess. I didn't really want to waste my time on him.
Zeno Clash, Stalker: Shadow of Chernobyl and Call of Pripyat. I know the Stalker games don't really have an official percentage counter, but I know I have done everything there is to do in those games, including all the quests (and if I haven't then I am really close).