Knock-knock! That would be the last sound you'll hear before your Windows Mobile OS crashes and you'll be denied access to the Messaging Center. According to the article on Winrumours.com, there exists some special sequence of characters, which can be sent to a Windows-operated phone in an SMS or even Facebook message and lead to such unpleasant consequences.
Thank God, Winroumours didn't go so far as to tell their readers what the sequence is, but some mysterious tweets allow us to conclude that it is a hard one to figure out.
Anyway, I'll try to =-)