Mismatched Serial numbers is just that. Serial Numbers on a note that don't match. Here is an interesting story in CoinWeek about a woman claiming she was at the FUN show recently and had received a $20.00 bill as change after a purchase. She noticed it was a Mismatched Serial number and then took it to the PMG table to have it authenticated. I don't know about the legitimacy of the story but it was a great find. https://coinweek.com/paper-money-2/pmg-grades-mismatched-serial-number-error-on-us-20-bill/ Here is the $20.00 bill - Newbie searchers, keep on the lookout for these type of printing errors! Here are some examples of some other mismatched numbers - (Not my notes) And they do carry a pretty good high premium. Upwards of hundreds of dollars!
Cool story, thanks for sharing. What makes it even more unusual is finding a mismatched serial number on a fairly modern note. Mismatched serial numbers were more prevalent in the 60’s through the 90’s and have almost disappeared since then. They make you do a double-take when first viewed. You could easily miss it. Some of my errors.