Some crafty counterfeiters have been going to great lengths to dissuade merchants from even testing their cash. These fraudsters even marked their phony dollars with regular light brown markers so it appears as though they have already been tested in the past. Outsmart or be outsmarted, invest a whopping $5 on a counterfeit pen of your own, they will last for thousands of tests and don’t be bashful; use it or lose it.

The two have allegedly spent at least 14 counterfeit $100 bills in
Natchez and four in Vidalia, Godbold said. He was not sure how many had
been used in Ferriday. Most of them were used at restaurants, gas
stations, Wal-Mart and one at Natchez Municipal Court.

The bills have been hard to find and the counterfeiters have done several things to make them look real.

For example, when real bills are marked with a currency testing pen,
the line shows up a light brown color. With the counterfeit money, the
line is black.

To get around this, the bills were already marked with a light brown
marker so that cashiers would not think to mark them again, Godbold
said. [source]