NATS Major Incident Preliminary Investigation Report [pdf]
nats.aero
2 threads
> While this request was being processed, the NAS received a message for a higher priority activity to be undertaken which resulted in the squawk code allocation being paused while the system processed the higher priority message. Switching between different activities in response to prioritised requests is a normal function of the system; however, when the processing of the squawk allocation request resumed, the software defect meant it did not resume correctly and the resulting output was corrupted.
Tl;dr - a concurrency bug / race condition