"Invariably someone puts a flight plan wrong and it borks the system"
"If the flight data processing system is down for more than eight minutes, the flight server alerts controllers that the data it is getting is stale."
So if one flight is well off its declared route, it can mean that it's eight minutes before the controllers go to checking the radar directly.
What happens if a second flight goes wild within that eight minute window? Does it get noticed?
Could terrorists exploit this by hijacking two aircraft? Use the first to crash the system by flying wildly, then the second has eight minutes before the RAF get notified.