Misprints with mismatched block letters are very rare. There are two block letters in each serial number. One letter begins the serial number and one letter ends the serial number. The misprint below shows a bill that has a serial number that begins with F on both serial numbers. However, one serial number ends with the letter L and one ends with a star symbol. Both should end with L or both should end with a star symbol. Because they are not the same, they are mismatched.
If the serial number on your bill has two different serial number block letters or symbols then it could be very valuable.