ohh.... my experience must be the worse,
I have 2 very bad exp in GT. The first is in GT2, one of my friends has erased my game, and I don't had a back up, at this moment, I had raced about 830 races and was the winner in about 800 of them, and the 100% was doned a whole year before this. I want to do it again, so I was going for the 100% again, and the new is so my save game today with over 2.300 races and a win ratio at 100.00% hehe
My second bad exp was here in GT4, I don't now why, but I lost all my cars, just one car back (alfa Romeo crap-car '99, ****!!!), when I was stading at 95,7% complation. I have my back up memory card, and it shoud opdates, and so the failere came, the day after I took the 100%, and now I've complated all the races for the second time hehe