Data entry can, indeed, be difficult to validate but in the case of an air traffic control system, you both can, and must, ensure that data entry is strictly controlled.
Yes, but do you know what went wrong and what needs to be controlled? Do you know how to validate a flight plan (I don't)?
For example, suppose, when the plan was entered it was fine, but some regulation changed later that rendered it dangerous. How would you have validated that?
Or suppose the flight plan was fine but some beacon it relied on suddenly had to be taken down for repair whilst the aircraft was already en route. Would you have thought of that? How would you validate such a flight plan given part of it became invalid after the aircraft took off?
This is a very complex system we are talking about and foreseeing every possible failure mode in advance is impossible. There is no practical way to eliminate every possible failure and certainly not by validating data in advance. The system failed safe in this instance, causing a lot of inconvenience, but it could have been a lot worse.
