What Would Likely Happen In This Scenario?

So as many of you know, I work in a pharmacy, and I'm likely the only employee there who doesn't use the counterfeit detector pen. My question for you is: What would happen if someone spent a small portrait bill from 1950 or earlier and it came up as fake on the pen test?

I really hope that if that happened, the cashier wouldn't believe the pen's false result and subsequently call the cops, I actually read an article of a man who was arrested for passing counterfeit money, until secret service took it to a bank to verify its authenticity and it was determined "real but very old." Goes to show you that people are getting arrested for the dumbest things, even when they didn't do anything wrong.