There is this migrated question on Transition Probability. Since the user has no Math.SE account, he's greyed out.
But funnily the only answer given is accepted. By whom? By the user?
Or was his Math.SE account deleted right afterwards?
Since the user who wrote the accepted answer does not have an account on math.SE, I assume that the acceptance was before migration. If the author creates an account on math.SE and associates their cstheory.SE account, they can decide to accept another answer if they wish.
If the answering account had been on math.SE and was removed after migration, the user would show up as userxxxxxx. So, as Anna Lear comments, the answer was accepted before migration.