https://en.wikipedia.org/wiki/Counterfeit_banknote_detection_pen#False_positives https://en.wikipedia.org/wiki/Counterfeit_banknote_detection... > The effectiv

https://en.wikipedia.org/wiki/Counterfeit_banknote_detection_pen#False_positives https://en.wikipedia.org/wiki/Counterfeit_banknote_detection... > The effectiveness of the pens may be affected by external methods. Simply having a banknote pass through laundry, depending on the soaps and bleaches used, can cause a bill to fail the test when it is otherwise accepted.