Rectification of mistake: Assessing Officer may amend orders to correct apparent errors, but must notify and hear before increasing liability. Rectification of mistake empowers the Assessing Officer to amend any order to correct an apparent error within the prescribed period, either suo motu or on the assessee's representation, by a written order. Amendments reducing assessment require the Assessing Officer to make any refund due. Amendments that increase liability, including enhancing assessment or reducing refunds already made, require notice to the assessee and a reasonable opportunity of being heard; the Assessing Officer must specify the sum payable and apply the Chapter's provisions accordingly.