The Mechanics of a Failed Checksum

When the actual 13th digit does not match the calculated Luhn digit, the number is structurally invalid.

Common Failure Causes

• Transposition Errors: Swapping two adjacent digits (e.g., 87 becoming 78). This is the most frequent typing error the Luhn check is designed to catch.
• Single-Digit Errors: Typing one wrong digit (e.g., a '3' instead of an '8').
• Data Entry Error: Misreading the number from a document or typing too few or too many digits.

Debugging Tips: If validation fails, check the date of birth (first six digits) first. If that is correct, re-enter the entire 13-digit sequence slowly, as mistyping any of the first 12 digits will result in a Luhn failure.