Okay, here's a fairly obscure one.
Normally, if you edit a post, then undo all your changes again within the grace period (manually that is, rollback doesn't work), the revision will be silently deleted.
However, if the last editor was Community, typically because the question was closed as a duplicate, then it doesn't happen, and an empty revision stays behind. Strangely enough, it has to be Community; anonymous editors don't count.
An example of these empty revisions can be seen here (revision 3, click the third button in the left).