It’s always best to stick to standard procedures as they are there for a reason. Back in the 80s there was a cracking Airprox (Airmiss) in the DXB hold between two aircraft waiting for weather improvement. An Air India B707 had been in the hold for a while at FL200 when a Heavylift Shorts Belfast freighter came on frequency at FL210. After holding for around half an hour the Belfast crew, unaware of the B707 almost directly beneath it, commenced descent. It was the middle of the night and with nothing else going on, the radar controller wasn’t paying attention to his screen but fortunately the aerodrome controller noticed the descent on his radar feed. By the time he alerted the radar controller the Belfast was just behind and above the B707 and was instructed to “Climb immediately FL210”. Unfortunately, inertia took the Belfast through FL200 before a laborious climb regained separation so there were two Airproxes for the price of one!
The reason that the Belfast had started descent without apparently getting a clearance was poor coordination or misunderstanding from Bahrain Centre. Standard inbound clearances were given FL130 and this had been passed to the Belfast crew but they requested to maintain their cruise level for the moment, thinking that they could “descend when ready”. The DXB radar controller, unaware of this, was told that the aircraft was maintaining FL210 to await WX improvement. Had the Belfast crew made a leaving call, the situation would have been avoided. (Of course this was pre-TCAS)