Bills can also get starch on them in various innocent ways. They get washed in the laundry a lot.
You can take a bill, spray it lightly with laundry starch, and watch it fail the pen test.
I didn't think about that one. Always a lot of money going through the wash. My wife and MIL have a habit of jamming folded bills into their pockets, and it winds up in the washing. We use $100 bills often, because things are expensive; most clerks don't even bat an eye around here, taking them without question. We try not to use credit cards, as that's a sure way to create debt.