Rectification of mistake permits officer to amend orders and requires notice before enhancing taxpayer liability. Section 74 permits the Central Excise Officer to amend orders to rectify apparent mistakes within a prescribed period, on own motion or when brought to notice by the assessee or certain officials, except as to matters already decided on appeal or revision. Where amendments increase liability or reduce refunds, the officer must give notice and an opportunity to be heard and issue a written order specifying sums payable; where amendments reduce liability or increase refunds, the officer shall effect the refund.