I just tried casting a delete vote on a question. I was given an error because the question was a target of a duplicate closure. In addition, the question closed as a duplicate was later merged with its dupe target. So we end up in a situation where the merge stub can't be deleted because it is locked, and the dupe/merge target can't be deleted because it has a linked duplicate.
I have to guess it's a fairly small number of questions that have been merged with a duplicate target which is then closed and is delete-worthy, but thought I'd report it anyway.