Errors can occur in trial balances that do not result in the debit and credit columns not agreeing.
Some errors revealed by a non-agreeing trial balance include omitting to post an amount, posting an incorrect amount, or posting to the wrong account side.
However, some errors are not revealed by the trial balance such as errors of omission where a transaction is not recorded at all, errors of commission where wrong amounts are posted on both sides, or compensating errors where one error offsets another.