Modern marker pens do not work on some American currency made before 1959 because the content of the currency changed that year. The printing process changed from wet-printed notes to a dry printing process and some of the wet printings do not pass the marker test.
Counterfeit detector pens work by using an iodine solution that reacts with wood pulp in paper rather than fiber-based cloth paper in American currency. A man in Tennessee was arrested in 2012 for allegedly trying to pass a counterfeit bill, but two banks verified the note was real even though it was old. Walmart's listing for detector pens states counterfeit pens may not work on currency dated before 1959.