One commonly used ICAO phrase which is ambiguous refers to leaving an altitude. For example ATC might say: XYZ Descend to 5000 ft. The pilot replies XYZ - 5000 - Leaving 9000. Can ATC be quite sure that the pilot has indeed left his current altitude for 5000 - and thus clear another aircraft to the supposed vacated altitude? Often we hear "Leaving" as the automtaic reflex reply to the ATC request when in fact the aircraft has not yet vacated its altitude.
Preference in Australia was to use the term "Left" the altitude as an unequivocal statement - rather than "Leaving" an altitude which leaves a lingering doubt as to exactly when the aircraft will commence descending. Unfortunately pressure to conform with ICAO terminology caused the Australian CAA to delete the term "Left" and substitute "Leaving".