Normally, in a post's revision history, users are able to roll the post back to any revision prior to the last one. The rollback link is hidden for the head revision because obviously it's pointless to roll back to the same, latest revision.
However the rollback link isn't hidden when there is another entry in the history that is not an edit, such as "Post Closed", above the last edit. Clicking rollback refreshes the page, but obviously does nothing meaningful since nothing is actually changed. Someone who's not paying attention might try to click it thinking it means "undo this edit", only to find themself staring at the same revision again.
Ideally, the rollback link should be removed regardless of whether there's a different revision entry following the edit.