Create more than one comment by the same user on the same work (top level comments, not replies). Then add replies to both of them. Try to delete both original comments - the first one should work, but the second one will fail, because when a comment is part of a thread, we mark it as deleted instead of actually destroying it. When we do that, we set the content to "deleted comment", but unfortunately, we won't let you save two comments with the exact same content on the same thing.
Fix: skip the uniqueness validation when the comment is being marked deleted. Then both comments should be able to be deleted.