The Federal Reserve has stringent policies in place to produce perfect notes. Printing currency is a highly complex process though and some error notes do escape to the general public.
Obstructed Printing: The seal and serial number is missing only from the right side.
Blank Back or Blank Reverse: The reverse printing is missing.
Offset Transfer: Partial portion of the back is seen on the front.
Gutter Fold or Blank Crease: The paper was folded prior to printing. This creates a gutter effect on the note.
Printed Fold: The paper was folded prior to printing.
Offset Transfer: The back is faintly seen on the front.
Misaligned Overprints: The second printing is misaligned.
Insufficient Inking: This series consisted of 27 notes having this error. See article below.
Article mentioning all errors of this type.
Reverse Overprint: The 3rd print is on the back instead of the front.