Looks to me like you issued a delete on a post that you then had refreshed (with the infraction for the next set), but the post was already gone. I would assume that's using the checkbox for all posts instead of just the one post. Its an oversight in the code for sure, but it doesn't look like anything to be concerned of.
Another thought that may make more sense. Perhaps you had already checked a thread, deleted via cascade, and then it recalled it when you tried again from the cookies with the checked items.