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!