Topic: Missing check(s) in moderate.php
Well, one check kind of leads to the other
Anyway, it's possible to delete the first post in a topic without deleting the topic (it took me tinkering with a Firefox extension I have to get it to happen though).
The second is that, because of the first, you can cause an SQL error (since it deletes the post and therefore the last_post info can't be updated) and have a topic with no posts in it (pointless, yes).