Some years ago in Australia, if ATC told you to (say) "Descend to flight level 130" you were required to report leaving your present level. The pilot would say "Left flight level 350" (the height he was currently flying at) Clearly there was no doubt in ATC's mind the pilot was actually descending since he had reported that he had departed 350.
Then that was changed because of an ICAO directive and the pilot instead would reply "Leaving flight level 350." This led to doubts that the pilot had not yet left 350. Or was it he had received the instruction and intended to leave 350 since the requirement in this situation was the pilot must vacate that altitude within one minute. So now the current situation exists that there is a modicum of doubt that the aircraft has physically vacated since the word "Leaving" signifies an intention but not necessarily that it has happened.
There were slack-arses who would also reply to an ATC instruction to descend to flight level 210 by saying "Flight level 210, left 350" when there was no way the aircraft had commenced the descent in the 3 seconds it took to reply.