Just got one of those new-fangled, redesigned ten-dollar bills? Don’t spend it yet! It could be worth something. Some of them have a serious error, making the bill worth over $30, a figure that’s expected to climb. Here’s how to spot it.

First, look at the serial number. The first letter should be "B". Then look at the second letter, and see if it’s "C". If so, there should also be a "C3" in black under the top-left serial number. This designates the third, or Philadelphia Federal Reserve Bank. If the bills is indeed from Philly, hold it up to the light and look for the watermark, a second, brownish portrait of Hamilton that’s identical to the main portrait. On normal bills, this mark appears on the right half of the bill, with Hamilton looking like he does in the middle of the bill. If the mark is instead on the left of Hamilton and upside-down, then you have the error bill.

It is not expected that many of these leaked out. But some did, and no one can be sure where or how many. Remember, they’re only on bills with the "C3" designation. Good luck!

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