In a forum I moderate for, a moderator was removed several months ago but they still have the moderator flair icon next to their avatar, and neither me (a moderator) nor a Forum admin can figure out how to actually remove this flair from them.
Our forum is currently on 3.6.0.beta2-latest and was just updated today to that, and the flair still remains.
You can impersonate the user and remove the flair. IIRC when a user leaves a group that has a flair, the flair stays. I believe it can be removed in the user preferences.