Traders in Falmouth have spotted fake £20 notes in circulation.
Nicola Willis, owner of the vegetarian cafe Pea Souk, had one of the counterfeit bills handed in just before Christmas.
When she tried to deposit it at the bank on Christmas Eve, the teller pointed out that it was a fake.
Nicola said: "If someone's trying to pass off a fake note, it's pretty mean to take it to a small business. Maybe the person who gave it to me didn't know."
Nicola joked that she might have liked to spend the note at a large retailer if the bank had not confiscated it.
She said: "You get nothing back."
The Bank of England has published advice on identifying counterfeit notes.
Key security features on the £20 note include a watermark of the Queen's portrait and a bright '£20' when held up to the light, a holographic strip with various multicoloured images on it, raised print on certain words, a red and green '20' that appears under ultraviolet light, a metallic thread running through the note, high print quality and micro-lettering in certain areas.
Devon and Cornwall Police have also issued advice on counterfeit cash including a video telling people how to spot a fake.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here