Police in Quincy, Massachusetts, are warning businesses about counterfeit bills that have been popping up in the area.
Over the weekend, police said there were three counterfeit cases reported.
"They will buy something small and try to pass a $100 so they get a large amount of cash back," said Quincy Police Sgt. Karen Barkas.
Cashiers are now taking a closer look when customers pay with cash and using counterfeit detector pens.
"We are taking more precaution for that," said Sam Patel of Quincy Market.
Authorities say the counterfeit bills aren't the result of a sophisticated operation.
"Some of the bills that we are seeing tend to be more amateur, it appears they are photo copied," said Barkas.
Some bills are cut unevenly and others have "for motion picture use" printed on them.
Put side by side, the difference between a real $20 and a fake one is even more obvious.
"You can also tell by the feel of the bill you know if you touch them they feel a lot smoother," said Barkas.
Business owners say during busy times when they're dealing with getting through a crowd of customers – paying close attention can be difficult.
"They know when to come, when there are 3 or 4 people in line or they'll try to tell you they're in a hurry like they have to catch a bus or something," said Patel.