Information from a European ANSP

• 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.