I have two.
My 150+ hour Oblivion save file. It wasn't lost... just corrupted. I could load it, take two steps, and it would freeze every time. Ick.
I lost 200+ Xbox achievement points because I had been playing offline, and didn't think about it when I proceeded to recover my gamertag on another Xbox. Among those achievements was the one for completing Halo 3 on legendary. So... I did it again.