Just lost my entire iPhoto library due to an ill-advised space-saving tidy up.
If you’re not an iPhoto user you can safely skip the next paragraph.
The old iPhoto used to store a lot of photos twice, so a user could revert to the original. These originals were stored in folders labelled ‘Original.’ If you knew you were happy with the tweaked photo, you could safely delete these folders and thus save a hell of a lot of space. But from iLife ’06, this seems to have changed. I only realised as my Trash was emptying and it seemed to be destroying thousands of photos rather than the usual ten or twenty.
Fortunately, I had a backup to revert to after a lot of mewling and whinnying and general unhappiness.
The moral: as always, BACK UP YOUR DATA!