Uh, Mel, I think we're mostly kidding around here. With all the surveillance equipment out there, it would be nearly impossible to do something like that without getting caught.
There are post-it notes and wiper cards that can be purchased but they're very diplomatically written.
There is a cool website that began as a service to the bay area in CA and it's taking off quite well. I'm involved in trying to get one through our state police here.
Check it out:
http://www.handicappedfraud.org/
Of course, there are times when only a baseball bat can resolve a conflict.