I looked into the thread you mentioned, to make sure everything looks kosher. I didn't find anything that would indicate a problem with the code, or evidence of hacking or anything like that. It does look like half a conversation, however, so my best guess is that the missing messages belonged to a user who decided to delete them later. I remember putting a time limit on editing and deleting one's own messages, although I can't remember what it is off the top of my head. Whatever it is, apparently he made it in time.
Regards,
Jonathan