Errors of commission are the errors that are committed due to wrong posting of transactions, wrong totaling or balancing of the accounts, wrong casting of the subsidiary books, or wrong recording of the amount in the books of original entry, etc.
For example, Raj Hans Traders paid Rs. 25,000 to Preetpal Traders (a supplier of goods). This transaction was correctly recorded in the cashbook. But while posting in the ledger, Preetpal's account was debited with Rs. 2,500 only.
This constitutes an error of commission. Such an error by definition is of clerical nature and most of the errors of commission affect the trial balance.