On 21 July 2023, a Boeing 737-800’s transponder failed during the cruise at FL 350 but the indication that it had done so was outside the pilots normal field of vision with no corresponding annunciation on the primary navigation displays or system status panels. Controllers in a low workload situation did not detect the loss of traffic from their displays and forgot about the traffic. The flight subsequently lost separation against an Embraer 190 which crossed in front of it at the same level. ATC collision risk detection processes and the 737-800 aircraft type transponder certification were both deemed inadequate.