Once you have closed the cash book ledger for a period it should mean you can no longer post entries into that  period. We had an entry post year end,  with a mistyped date  post back into period 12 of our financial year end and so the trial balance was incorrectly updated. Either an error message should come up to say the period is closed, are you sure you wish to post this entry or automatically push it in the next period as with the purchase/sales ledgers?