Air Traffic Services Overview

During a flight, if the aircraft's true airspeed or Mach number at cruise altitude differs from the planned value by at least how many kilometers per hour, must the pilot inform the appropriate air traffic control unit?

An IFR clearance includes the note: Clearance is valid until 0920. What action is required if you remain on the ground past 0920?

After extensive instruction and practice, what level of human mistake occurrence is generally regarded as acceptable and indicative of good performance?