On Tuesday, September 8, 2026, UK air traffic control lost its core flight data link and stopped departures from British airfields for about four and a half hours. More than 2,000 flights were delayed, cancelled or diverted. The cause, according to the preliminary report by NATS , the UK's air traffic service, was a legacy software defect with an exposure window of about one millisecond. Programmers have a name for that kind of bug: a race condition. This is the postmortem, read the way a devel...