This particular note I just listed for sale is fun, what's called a "flipper". It's a 2021 (most current) Series with the serial number composed only of 0s, 6s and 9s such that it reads as a number upside down or right side up.
For only being the three numbers used it is also what's called a "trinary". Had that "0" been a "6" this note would double or even triple in value for it. Had it been a "9"? Wow... 10 or 20x.

https://www.ebay.com/itm/137418959578
See all my notes for sale:
https://www.ebay.com/usr/mpoloukhine