Visa Reason Code 12.4

The transaction was processed against the wrong card account number.

A data-entry, card-on-file, or integration error applied the charge to the wrong card number.

Almost always a processing/system error rather than a customer-facing issue you caused directly.

Your system's record of the account number captured at checkout versus what was actually charged.

Manually re-keying card numbers instead of using tokenized, validated card data.