Hi,
most systems come with 2 or more transmitters. If You are good friends then it might be possible to 'borrow' one from Your neighbour. Wait until he opens the garage door and close it with Your transmitter always when it's half way open. Doing so with only one member of Your neighbour's family (for the other people the door works normal) can be great fun...
But as with every _good_ joke tell the person that he/she was the victim of a joke and give him/her a chance to laugh too.
(could be 'candid camera'-stuff)
regards
joachim