One of the many things that just drives me insane is when I go into a store, pay with cash, and they whip out one of those useless counterfeit detector pens to make sure I haven't given them funny money. Suddenly, I've turned into John Freakin' Dillinger armed with a $20 bill. Give me a break... if I'm gonna turn to the dark side at least give me credit for having the initiative to pull off a crime that's a little more ambitious than passing low denomination currency. I have to bite my tongue when this ritual takes place, since I recognize that I'm dealing with an hourly worker who is just doing what they are being told to do. My temptation is to buy a pen of my own, and to painstakingly mark and examine each bill that they give me in change, in an effort to slow down and back-up the checkout lane.
The pens (in theory) use iodine which reacts with the starch in normal wood-based paper that is used by counterfeiters, making a black mark. When applied to fiber-based paper used in real currency, there is no reaction. This handy little fact inspired another frustrated shopper to come up with a better idea for frustrating cashiers
: treating $20 bills with starch so that they register a false positive for counterfeits. But his experience with the less than amused members of the law enforcement community makes me think this may not be the best way to go.