Don't rule out the smart-azzed teenager possibility...
This. It is possible that someone found one, or was given one, and thought it would be funny to tip with it.
Christians have done some embarrassing things in the name of witnessing. Too many churches give people the idea that they must actively proselytize strangers on a regular basis. So people find themselves trying to do it in all sorts of clumsy ways. Of course, it sometimes does help; just not in the fake money as a tip sort of way.
