Answer
Occasionally the bank makes an error (wrong amount, wrong account, duplicated entry). Such items are not corrected in the cash book; instead they're shown as reconciling items and queried with the bank, which issues an adjustment. The reconciliation notes them until the bank corrects its records.
Bank debits 50 in error -> list as a reconciling item and query the bank.Real-world example A cheque debited twice by the bank is flagged in the reconciliation and referred to the bank to fix.