A Wizz Air Airbus A320-200, registration HA-LPM performing flight W6-3763 from Craiova (Romania) to Madrid,SP (Spain), was enroute at FL310 about 110nm east of Madrid when ATC cleared the aircraft to descend to FL300.
An Air Europa Express Embraer ERJ-195, registration EC-KRJ performing flight UX-1084 from Venice (Italy) to Madrid,SP (Spain), was enroute at FL290 110nm east of Madrid on the same airway as W6-3763 exactly below the A320.
The A320 descended below its cleared flight level 300 and reached FL292 before climbing again.
Spain's CIAIAC reported the separation between the two aircraft reduced to 200 feet vertical and 0.2nm horizontal. When ATC noticed the A320 descended below its cleared level, ATC immediately instructed the aircraft to climb. The CIAIAC did not report any advisories by TCAS. An investigation was opened into the occurrence.
After being clear of conflict both aircraft continued to Madrid for safe landings without further incident.
In December 2019 Spain's CIAIAC released their final report ...
The CIAIAC concludes the probable cause of the serious incident was:
The loss of separation occurred because the crew of the Airbus received an erroneous instruction with respect to the flight level to descent to.
Contributing factor was the saturation of the control frequency due to many aircraft contacting ATC at the same time causing communication difficulties.