Bank reconciliation is a periodic process to match the bank statement against the information you have recorded in the accounting system. Besides being a fraud control tool, it picks up error such as omitted entry, an incorrect amount recorded, duplicated information has recorded, has recorded transaction into a wrong financial period, etc.
Assuming you invoiced Customer A and Customer B with the same amount $321. Then, you accidentally knock off the payment received from Customer B against the Customer A's outstanding invoice. You can use the cancel transaction function to reverse the entry and enter a receipt from Customer B to correct it.
Alternatively, you can also use the 'undo' bank reconcile method if there are many mistakes made in the reconciliation. From the Bank Reconcile wizard, click the 'load old' button and select the previous bank reconciliation. Then, click the gear icon at the bottom of the wizard and select the 'clear all reconciliation from this one...' option. Assuming you have reconciled from January to June, and discover a mistake in March. You have to undo the bank reconciliation one month at a time from June to March.