I have seen on the recent 6.7 beta versions and prior, when putting in an invalid reply_num on the edit page, it passes through though the message box is blank.
A bug that got lost. Oops! Not a major issue, as the submit of the edit doesn't do anything harmful, from what I can tell. This will be fixed in 6.7 final. I'll post the rv number when I actaully commit the fix.