The police department in Media released photos of the bogus $100 bills.
Spotting the bills is a challenge, however, because they do not react to a counterfeit-detection pen.
"They are real bills. They've been washed down and reprinted," said Lt. Rick Johnson of the Media Borough Police Department.
One thing to watch out for: all of the counterfeit bills have the same serial number:
If it reads E 05201634 A, then call police right away.
In one instance, the suspect got change in real currency, leaving a shop owner out of cash and holding a phony bill.
"They're going to get 90 dollars or so change, and that's how they make their money," said Johnson.
Mary Eileen Maginnus taped a warning sign next to the register at her home store.
"One hundred dollars is a hundred dollars, especially when you own a store and you're making it day by day," said Maginnus.
Police say to look for any watermarks that don't match. On one bill, Ben Franklin could be seen on the front, but the bill had a watermark showing Abraham Lincoln.
Authorities recently busted a large ring of counterfeiters operating in the area but don't believe this is connected to those arrests.
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