There was still enough automation in those days to make errors. Consider KAL007 in 1983 when a simple mistake concerning the autopilot and INS inputs had catastrophic implications.
As I understood it (correct me if I'm wrong) KAL007 deliberately cut the corner as KAL had done for some time in order to save fuel.
On that fateful day, the Russians were conducting tests at Kamchatka Base and were on high alert, hence the response.
In other words there was no entry error. The INS story was a cover-up