I think the best one I have seen (though it wasnt mine personally), but I saw the message.
DL99 - Operational Incompetance.
This was used, after a 4 hour delay on a (now defunct) charter company which could have had any of 20 different delay codes for everything from tech aircraft to baggage discrepancy, aircraft swap, incorrect flight plans etc....
It was sent, and the airline questioned it, and demanded the correct delay code. Which was then given as the mobile number of the captain who suggested it, no more was said!
RTG!