[JetSetIlly]

There are several categories of mistake that you can make when bookkeeping. Some are caught by the double-entry system when a trial balance is prepared.

The error you've described is an "error of original entry" and will be invisible if you only look at the trial balance. It can ultimately be caught when you compare the banking ledger with what's actually in the bank.

Other errors that don't appear in the trial balance can be incredibly hard to detect and in fact, may never be noticed. This is where the real art of bookkeeping is IMO.

The types of errors that do affect the trial balance are things like forgetting to enter a purchase in the purchase ledger but entering the transaction into the banking ledger correctly. Silly errors really, but we can all need help to stop us making those.