In the month of March-18, we have two invoice of outwards supply. We have no any Input tax credit, so all liability is paid through Cash only, hence we credit same amount to cash ledger before due date of that month. But while filing of GSTR-3B,erroneously we show only one Invoice and liability is discharged by using cash balance.As one invoice is not reflected in 3B,we same amount of cash amount is lying in Cash ledger. As This may be added in next month 3B and liability is discharged from cash ledger then we have to pay interest. As we already paid cash in due time of respective month.