Rectification of mistake: authorities may amend tax records to correct apparent errors, with notice before increasing liability. Section 154 empowers specified income-tax authorities to rectify mistakes apparent from the record by amending orders and certain intimations, either on their own motion or when notified by the assessee, deductor, or collector, subject to limits where matters were decided on appeal. Amendments increasing liability require prior notice and opportunity to be heard; written orders must record amendments. Reductive amendments trigger refunds by the Assessing Officer; augmentative amendments trigger a prescribed notice of demand deemed issued under recovery provisions. Temporal limits and a statutory decision period for applications constrain amendments.