Recently the image Great_Seal_of_the_United_States_(small).png vanished; its image description page listed it as present and had a link to one revision, but the image itself was gone.
After brief investigation, I found that the image file was still present in the archives directory, and copied it back to where it belonged... but there had also once been an earlier revision of the same file (a non-transparent PNG), now missing both from the archive and the image page.
Grepping the access logs, it turned out that a spider had come across the image description page and followed links to both revert and delete the older revision of the image -- simply loading up these links caused the wiki to move and permanently delete files.
Apparently telling it to both revert and delete the same revision confused the poor wiki, and it ended up vanishing that revision entirely _and_ leaving the newer one only in the archives.
As a workaround until a better way of handling these functions is decided on, I've hacked Skin.php to not give the delete/revert links to anonymous users (and therefore bots and spiders).
-- brion vibber (brion @ pobox.com)