Well, I recently got a 5 digit MCO from Air Canada. Sadly, 2 of the digits were the cents ($500.00.) Still...

A while back I purchased an MHD type ticket that was issued on paper. After completing half the trip (and while still holding the paper coupons for the return portion) the entire purchase price was refunded to my credit card for reasons that astound and confuse even me. But as far as the airline was concerned, so long as I still had the coupons the ticket was valid. I used the last of it a few months ago, after making some changes which required me to mail the ticket to the airline, and wait for its return. Not an MCO, but neverthelss, a free ~$1900 ticket.