Jake Vinson's story of the punch card reminded me of my own. I was a member of a "negative option" book club. I had to send a punch card back every month to prevent an automatic shipment of the current selection. According to the instructions, if I wanted to terminate my membership in the book club, all I had to do was write "cancel" on the card and send it back. I did that for three consecutive months and the cards kept coming.
Staring in frustration at the fourth unwanted card, I noticed that only columns 1-10 and 71-80 were punched. The fields were account number and date with no check characters. Therefore, the middle sixty columns were all mine. I took the card to work, put it in a keypunch, and in those empty columns punched, "SO CANCEL MY MEMBERSHIP ALREADY!", with top edge printing turned off, of course. I don't know what that card did when it hit the book club's system, but I never heard from them again. Mission accomplished.