Pre-flight Important, Too
In Aug 1983, a CAL DC-10 took off from LAX headed for HNL. By the time they reached the departure gateway, ATC advised they were north of course. They checked the dual INS, and returned to LAX, flagging both INS for bad navigation.
That nearly state of the art INS had only one item in NVM, the lat/long at shutdown. That was same as departure lat/long in this case. Supposedly the FE had entered the departure lat/long, then verified by the pilots. The Lat was correct for LAX, but the Longitude was for Denver.
If they hadn't been caught by ATC, they may have ended up lost beyond VHF range of HNL, 800 miles to the west.
KAL007 happened a month later. Instead of 149W, the FE could have entered 140W or 139W.