We've recently gotten a flag on Stack Overflow indicating that a user has gone through and unaccepted every single one of their answers.
While this is perfectly OK and allowed by the system, it could be an indicator of vandalism, and right now, it's not something that's easily detectable.
That said, I'd like to see two things. First, there should be a way to see that a user unaccepted an answer in their activity. We can see accepts, but not unaccepts.
Note that we can look at the reputation and see the -2 applied for unaccepting an answer (and we can see a bunch of them in a row), but we have a specific tab in the activity section of our profiles to show our accepts, there's no reason we shouldn't have the same duplication of information for the unaccept case.
If a user unaccepts a number of answers in a short period of time, I'd like to see a flag in the moderator queue indicating that there were a number of unaccepts in a short period of time, just like we have for edits and deletes.
Finally, as per Sha Dow Wiz Ard's suggestion in the comments, there should be a rate limit on how many unaccepts you can do per day, as per the delete rate limit.