..... and a table for your dinner.
As Capt. JFK - LHR, and having "given" the flying for this sector to the F/O, I decided to put the S.O./ Nav in my seat as we flew up the East Coast towards Gander - this was an early INS trip when we still carried a qualified Nav. Pilot, "just in case", tho' he had normally nothing to do - so I enjoyed my meal at a table instead of balanced on my knee, made it easier to eat the caviar, should the cabin crew have been so kind !
I then noticed that the aircraft was in a positive turn to the right, one does notice these things, and heard the F/O explaining to Gander ATC that "we have a Nav failure".
The early INS would only accept 9 waypoints, and one had to constantly leapfrog from ones passed to insert new ones into the system ahead of the aircraft.
The F/O and S/O occupying the two front seats had been so busy chatting that they had forgotten to do this, so the INS had performed as designed and upon passing waypoint 9 had set course for waypoint 1 - back towards New York !
(Expletives deleted )