On my Stack Overflow activity page, there are seven "revised" entries that don't link to any post:
Thank you, Jon B, for the beautiful freehand circles.
Looks like a side affect of Post Type wiki. I will fix up the links tomorrow so it links to the tag wiki :)
It happened to me in light of the new tag wiki revision. When I wanted to look at a tag wiki as the first user after the revision feature had been included, the tag wiki migrated to the new structure. It looks as if for all revisions there is a recent activity entry generated when the tag wiki is ported.
Look at this entry as well, especially waffles answer.