There are actually very few. Usually illness contributes to it if I?m stuck to the couch for a week or so like with Assassins Creed 2. I?m not sure if I (technically) got 100% but I got platinum on it. Fallout 3 was just such a compelling game that I didn?t just play through it, I played it to death. Once again, I don?t know what 100% is in that game or I got it but I?m pretty sure I?ve done everything in that wasteland I can do.
San Andreas actually had a 100% stat I reached (the biggest problem were those Zero missions) because I just found that game so damn engrossing I couldn?t put it down. What helped is that all the little things that contributed to 100% gave you bonus toys: collectables gave you arsenals, r3 missions gave powerups and even the schools gave some of the best vehicles: like the hunter with all gold flight school.