Rectification of mistakes: authorities may amend orders but must notify and hear the taxpayer before increasing liability or reducing refunds. Section 17 allows authorities to amend orders to rectify mistakes within the statutory period; amendments increasing liability require prior notice and a reasonable opportunity to be heard. Amendments must be in writing. If an amendment reduces assessment, the Assessing Officer shall refund any amount due; if it increases assessment or reduces a refund already made, the Assessing Officer shall serve a prescribed notice of demand specifying the sum payable.