The question How to tab between buttons on an Mac OS X dialog box, recently migrated from SO to SU, has the interesting sight of two accepted answers. Even if this is a case of a merge (which is not documented on the page), surely the checkmark should only be shown for the answer accepted by this question's author.
I suspect this is an ancient bug (and probably long-ago fixed so new posts can't cause this anymore), but it's bizarre
The SO question was one of the first posts, and the timeline shows that it's always had two accepted answers, with no unaccept. The first one is the one recorded in the data dump as the actual accepted answer. The SU migration automatically unaccepted the second one, but both posts are still showing up with checkmarks, and the supposed unaccepted one is the one getting credit (this versus this)
The way the database schema is set up, only one post can actually be accepted, so the double checkmark has to be generated some other way. It looks like https://superuser.com/a/473144/36115 is the one that's actually accepted