How did KAL 007 get so far off course? Regarding Flight 007 that got shot down in the Soviet Union, this was reported in the New York Times:

Study Says Korean Airliner Was on Its Intended Course When Downed in '83 (nytimes.com)

Excerpt:


  A retired airline navigator has gathered evidence that in his view
  shows that the South Korean airliner shot down over Russia in 1983 was
  on a deliberate flight path and did not stray accidentally into Soviet
  territory.
  
  A principal part of his analysis is his refutation of what has been
  the prevailing theory, that the plane unintentionally flew off course
  because of a single-digit mistake in keying flight coordinates into
  the navigation system.
  
  Leading aviation officials accept the refutation as correct, but still
  insist that the incursion was accidental and not deliberate.


We know this was a modern (at the time) Boeing 747 with a complex inertial flight management system.



No, I do not believe the theories about this being intentional.  Nobody would risk a plane full of passengers like this on purpose.  

In 1978 the KAL flight 978 went astray and was forced down on a frozen lake inside Russia.  

Korean Air Lines jet forced down over Soviet Union - Apr 20, 1978 (history.com)

I would think this would be on their mind as they prepared for the flight.



So my question is:  How did KAL 007 get so far off course?

Follow-up:  Wouldn't they have noticed?
Chapter 4 of Degani's Taming HAL covers some of the details of this disaster, and is on a NASA site The Crash of Korean Air
Lines Flight 007

It details some of the interface deficiencies in the aircraft's navigation selection system.  We don't have definitive evidence about exactly what happened, but it is possible.  It goes over the idea that it was easy to get the system in a mode where the pilots could easily have thought that the inertial navigation system was engaged, but actually wasn't.  

The Mayday show covered this one as well.  They noted that after the FDR was returned, it showed that the FMS was set properly.  Even though it wasn't controlling the plane, it would have continued to show the waypoints being passed as the aircraft crossed to the side.  Given that behavior, it would be very easy to believe that the system is working and you're where you should be.