Example scenario:
say, user can't post without approval (Newly registered user/etc.).
1) User post a topic/message.
2) Moderator approves the message.
3) Someone replies to the message.
4) User edits the message approved, the message goes to the moderation queue again.
5) Moderator disapproves the message.
6) The topic has the reply to the deleted message only.

That looks a kind of unlogical. Probably, either users who can't post without approval should be disallowed to edit replied posts, or such a message shouldn't be allowed to be disapproved once it was approved before (or shouldn't go to the queue again even).