Inbound aircraft is instructed by sector A to contact the next sector B.
Pilot doesn't establish contact with sector B and realises the problem
Pilot looks up a frequency for the area he/she is in (on his/her charts), and happens to select the frequency of sector C, subjacent to sector B. This sector happens to expect the aircraft to come to them from sector B after the latter has descended the aircraft to the coordinated level.
Pilot fails to explain the situation (i.e. lost correct frequency) and doesn't report actual cleared level etc. Sector C assumes the flight was transferred to him by sector B and issues a descent clearance into his airspace.
Aircraft descends through the traffic of sector B, without any separation assurance from that traffic. Sector B is left with trying to second guess what the flight is doing.
Incomplete initial calls from the aircraft (e.g. only stating the call sign) & not explaining they were unable to establish contact with sector B.
Failure of sector C to recognise the unusually early transfer.
Sector B doesn't realise that aircraft is lost and has no time to clear aircraft in its vicinity.
Radio communication failure procedures are provided in ICAO Annex 10, Volume II, 18.104.22.168.