My personal favourite was sending data in outgoing emails to invalid destinations. Your data would get queued and a few days later you'd receive it back in an

My personal favourite was sending data in outgoing emails to invalid destinations. Your data would get queued and a few days later you'd receive it back in an undeliverable-message. However this stopped being effective when mailserver stopped sending back the complete body of the bounced mail. Instead just a "Your mail couldn't be delivered, blah blah". You could deal with that by encoding your data in the subject-lines, though I guess ..