Some questions I have about counterfeit money ...

Specifically, I want to know more about those little magic markers that cashiers use to scribble on any bill larger than a 20. Evidently, the scribble is supposed to turn black if the bill is fake. So ...

First question: What would happen if they discovered that it was actually counterfeit money, passed unwittingly on to me by another merchant? Could I be arrested? Has this ever happened to anybody?

Second question: Don't you feel a little insulted when they do this in front of you? And is it my i