20d... Involved a BA -400, and I'd spoken to one of the crew. As you say, the other aircraft (cannot remember airline involved) had a Alt/ADC => TCAS wiring / logic fault so reporting wrong FL. Was v frightening, as you can imagine, since the "conflict resolution" very nearly actually caused 2 aircraft well separated to collide.
I cannot remember his words, but it was pure fortune, maybe some lateral separation (but very little), and maybe slightly slow reactions on the part of the "other" aircraft, as well as becoming VMC that "saved" the day. Exemplary following of SOPs by both crews could have led to a major (2 x widebody) collision.
However, it was investigated, was sometime ago, and one would hope such an "own goal" has been designed out (as suggested earlier with the checking systems).
I always find it ironic that the light aircraft I fly show on the panel the FL beng pushed out by Mode C.... no airliner I have flown does
