Balances on both sides of a control account ??

It may happen that there is a small opening credit balance brought down on sales ledger control account , along with the usual opening debit balance . Why ? for example , if a has returned goods after paying his account or a debtor has overpaid his account . Similarly , it may happen that there is a small opening debit balance on a purchases ledger control account ,along with the usual opening credit balance . If the business has returned goods after paying the account or where a business has overpaid its accounts the purchase ledger control account will show a debit balance.