Which of the following errors affects the agreement of a Trial Balance?

A trial balance lists the closing debit and credit balances of every account, and it will balance (agree) as long as total debits equal total credits. Some errors disturb this equality; others do not, because they still leave debits and credits equal even though the accounting records are wrong in another sense.

A wrong addition (casting error) in the sales account changes only that account's total without a matching, offsetting change elsewhere, so total debits and total credits no longer match; this is the type of error that is caught by the trial balance.

The other errors listed are all errors that leave the trial balance balanced: posting the purchase of a van to the purchases account debits the correct amount to a debit-side account, so the totals still agree even though it is really a fixed asset, not a purchase for resale (an error of principle); failing to enter sales in the books at all omits both the debit and credit sides equally (an error of omission), so the totals still match; and crediting a purchase to the wrong personal account (A. Tambi's instead of F. Tambi's) still uses the correct amount and side, just the wrong person's account (an error of commission), so total debits and credits remain equal.

The key distinction to remember: errors that affect only one side of the double entry, such as a wrong addition, break the trial balance's agreement, while errors of omission, principle, and commission typically do not, because both the debit and credit sides are still affected by the same, correct amount.