These are colloquially known as "gas pump errors".
Cause: These are generally caused by errors during the third printing pass (serial numbers and seal) or a misaligned overprint.
Types: This can include a single "stuck" digit (one digit is higher or lower than others), multiple misaligned digits, or the entire serial number string being shifted.
Value Factors:
Minor "Gas Pump" Errors: These are common in high-speed printing and often carry little to no premium over face value.
Major Misalignments: If the number is significantly misplaced, touches other elements, or shows parts of another bill, it is a collectible error, potentially worth hundreds.