I've always had a save file phobia, fearing someday...
... the console I was gaming on would betray me the morning after and show a clean slate of EMPTY SLOTS as I tried to load back my game (i.e. I mentioned this in another thread: my GameBoy went Alzheimer's on my Pokemon Gold)...
... or that I would accidentally save past the "point of no return" (i.e. the Ark of Yamato in Okami)...
... or that a friend I was having over would record his progress over my progress (i.e. I played Mercenaries: Playground of Destruction the whole Summer before my cousin saved his insipid progress over my file)...
...or that yours truly would make the same idiot mistake (i.e. erased my Sora endgame with my Riku file in Kingdom Hearts re: Chain of Memories).
Until I learned to emulate a GameBoy, save on several slots, and swipe Memory Cards in between having a friend and playing a game on my own.
Title question. Worst saving accidents/mishaps? Harder as this is to do on the current generation, I'm sure we all have Memory Card days.