How did KAL 007 get so far off course?
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.