It was last night, actually. I started the game, as always, but it crashed in the loading screen (where you run around with Desmond, but not throw sand...). I booted up the game again and my save had been corrupted, completely unsalvageable. And uPlay had backed up my save to the cloud after it had become unusable... awesome.
I didn't lose much progress, though. I was just starting Sequence 6, but now I have to redo the 10-hour prologue. Which means I have to play as Haytham and redo all the petty missions until Connor finally becomes a full-fledged Assassin. And that's grand.