A very popular paper money misprint is the completely missing face error. These misprints occur when the second print on a bill is missed. It results in something like you see below. The portrait and all the black design is missing.
These errors are especially interesting because it is almost impossible to tell what year the bill is from. Sometimes you can use serial number ranges to figure it out. In most cases the year can never be determined.
Bills without a face are usually worth somewhere between $200 and $500. The low end would be for a note in circulated condition and the high end would be for the same note in perfect condition.