A passanger plane actually collided into another plane at cruising altitude a decade or two back because ground control made a mistake in ordering a plane to change altitudes.
Are you referring to the DHL-Aeroflot (or some Russian carrier, it was a Tupolev) midair collision that happened over France IIRC? If so, that was due to a combination of errors, as most aviation accidents are as I am sure you know.
Anyway, the controller was alone and not paying enough attention, and was prosecuted for it. The two airlines TCAS systems gave the pilots adequate warning and proper instructions to avoid collision. But the ATC, finally noticing at the last minute the two were going to collide, gave the opposite instructions of the TCAS to the two pilot crews.
The pilots decided to listen to the ATC, and collided midair. Had they listened to the TCAS, they would have avoided the midair collision.
All lives were lost of course, and the Russian airliner was carrying something like 80% young students on their way to some cultural exchange.
Sad event, and now pilots are instructed to always listen to the TCAS over ATC instructions from what I understand.