Trojan horse

When you smear poop all over the top of a toilet seat (stress only the top of the seat) and than place the seat in the up position. The stall must be otherwise clean to give the next user the impression that someone has left the seat up as a 'gift' to keep it free and clear of urine. When the next user attempts to move the seat into the number 2 position he/she finds that the 'gift' is a Trojan horse.