Rectification of mistakes apparent from record permits authorities to correct record errors within prescribed time limits, subject to natural justice. Authority issuing a decision, order, summons, notice, certificate or other document may rectify errors apparent from record on its own motion or when pointed out, subject to time limits including a general six month bar and an exception for clerical or arithmetical slips; natural justice must be observed when rectification adversely affects any person, and substantive parts of instruments cannot be amended by rectification.