I've just read a post called How to crash an in-flight entertainment system, in this post the author (a security consultant it seems) reports how managed he crashed an inflight enternaiment system, by just fiddling with input values. A rather nice story, that should be really called, the importance of unit testing. :-)
With effective unit tests that performed some boundary value testing these kind of situations would be easily caught during development time. I only hope the rest of the airplane software uses unit testing. :-)