Any time you see a serial number or seal in a crazy place on a bill, it is likely the result of a foldover error. Foldover misprints occur when the sheet of currency is folding during the printing process, thus resulting in some part of the bill being printed somewhere it shouldn't. The note below has the seal, serial number, and series year on the back of the bill.
Foldovers are some of our favorite misprints because anything can happen with them. They can split serial numbers in half, or they can completely cover entire printings.