The following episode occurred on December 20th in 2002, but was not made public until recently.
At around 8PM, while a Norwegian 737-300 was flying from Oslo to Bergen, a SAS 737 was making it's way from Bergen to Oslo. As the two aircraft were about to meet over Sigdal, the ATC decided that the Norwegian aircraft should level out at 28 000 feet. The SAS plane was cruising at 29 000 feet in the opposite direction.
However, the ATC forgot to give the instruction to the Norwegian crew, who were originally cleared to 30 000 feet.
It was not until the SAS crew reported that the oncoming traffic was still climbing towards them, that ATC realized what was about to happen. At this point he ordered the Norwegian flight to "descend immediately to 28 000 feet."
The anti-collision system in the SAS aircraft was activated, and commanded the crew to descend and turn left. The SAS crew followed these instructions.
However, the anti-collision system in the Norwegian aircraft also became active, instructing the crew to climb. The pilots however, decided to follow ATC orders to descend to 28 000 feet.
The result of this was that both aircraft began descending from 29 000 feet simultaneously. At 28 100 feet, the aircrafts passed each other separated by only 2 NM.
The commission which looked into this episode and later released the report, have mentioned the mid air collision on the border of Germany and Switzerland in July 2002.