A potential windfall could be on the horizon for Americans and holidaymakers with leftover money as currency collectors scramble to find seemingly ordinary $1 bills that contain a printing error.

The notes contain an error from the US Bureau of Engraving and Printing, making them worth more than their face value.

An estimated six million of these notes are in circulation.

Chad Hawk, vice president of PMG, a professional paper money grading company, explained that it’s unusual for such errors to slip through the Federal Reserve’s checks and enter circulation.

He told Fox Business: “You just don’t see that error in US currency.”

Two batches of these $1 bills were printed – one in 2014 and the other in 2016 – resulting in over six million flawed $1 bills now in circulation.

He explained: “In 2014 and 2016, there were orders sent by the Federal Reserve to facilities in both D.C. and Fort Worth to go ahead [and] print the [same] serial numbers.

“So all the notes were $1 bills from New York [in] 2013, which, for your normal circulated currency in your wallet, would just be a normal $1 bill.”

Hawk pointed out that the issue surfaced when these dollars were printed with duplicate serial numbers.

Each dollar bill in circulation is supposed to have its own unique serial number for identification purposes.

In total, there are approximately 6.4 million pairs of $1 bills with matching serial numbers.

“The community, through social media, has been able to connect – and people have been able to pair up their notes in a lot of ways. The last pairing I think I saw sold for about $6,000,” Hawk revealed.

Despite billions of dollars being in circulation, only nine pairs have been discovered so far.

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