Last modified: 2011-07-18 02:37:04 UTC
If you look at http://en.wikipedia.org/w/index.php?title=Crab&action=history you can see that two edits are switched. An anon makes a vandalism edit, and then was reverted by AntiVandalBot. But in the history, AntiVandalBot is mentioned first, and then the anon vandal. This gives the following two edits: http://en.wikipedia.org/w/index.php?title=Crab&diff=83283439&oldid=83049368 (av-bot) http://en.wikipedia.org/w/index.php?title=Crab&diff=83283429&oldid=83283439 (anon) As you can see, the antivandalbot edit is shown as empty, but looking at the current article it is not vandalized, as it should be if the last edit (the anon vandal one) shown in the article really was the last edit.
Looks like some server was off in time
Not a bug in MediaWiki, the only possible issue is that we trust that systems are synchronized. I know that there are other related bugs - I'd mark this duplicate if I could find any - but this is old enough that everything's probably been improved long ago anyway.