- They had two links from their System Flight Server, one for redundancy if one goes down. Both went down at the same time, apparently unprecedented for them.
- There was a system limit for a shared system resource (Atomic Functions) that was defined twice in different systems, but with different magic numbers. The problem wasn't spotted earlier because a recent system change actually brought one of the systems close to the limit for the first time. (More military controller functions were amalgamated into NATS the month previously)
- There was a UX problem where the "Select sectors" button, which is used often, is placed directly beside the "soft Sign off" button, which isn't often used, and in fact was well known to be pressed by accident relatively often. It was pressed at the time of the incident, putting the system into an illegal state and hence triggering an automatic shutdown.
Problems that on their own you could argue aren't showstopper problems, but when triggered together cause things like this.