"As soon as I got to it I dropped it and said, `This isn't real,'" Piper said, adding that after 16 years in the business, she can tell whether money is fake just by the feel of it.
Since the fake bill was discovered, Thomas has taken it around to local business owners as a tool to warn them to be on the lookout.
Piper said she's come across three counterfeit bills in the past three years.
To the novice money handler, the $20 bill in question seems real enough.
But Piper pointed out several flaws in the note.
Compared to a real bill, the counterfeit one is slightly smaller in size. It also has a greenish tint to it that suggests the ink is beginning to run.
The counterfeit bill doesn't have the magnetic strip embedded into the fibers, or the telltale ridges across the bottom of the portrait that are obvious to the touch.
Most banks have a counterfeit detector which looks like a highlighting pen. On a real bill, a mark from the pen doesn't appear. But the same mark on a fake bill turns a purplish-gray, Piper showed.
To help protect U.S. currency, the U.S. Department of the Treasury recently issued new $100 bills with enhanced security features, including a polymer thread embedded in the paper that indicates the note's denomination by its position.
Other security measures on the new $100 bills include micro printing, watermarks, concentric fine lines and color-shifting ink, according to a poster released by the U.S. Dept. of the Treasury.
But with today's advanced technology, such as color copiers and laser printers, Thomas said it's not hard to pass a fake bill.
"It's a federal offense if you get caught," he warned.
Anyone who suspects they have a counterfeit bill in their possession should turn it in to police.
"Do not try to pass it off to the next person," Thomas said. "Charges can't be filed against you if you bring it in."
McConnellsburg State Police investigated the passing of two counterfeit $20 bills to local businesses early last fall. The McConnellsburg and Mercersburg cases have been turned over to investigators at the United States Secret Service.