Easy mistake to make. If I go back to the B757 it didn't have GPS so the position was updated via DME/DME or DME/VOR radial. Prior to departure the airfield code was input into the FMC position update which then the appropriate (displayed) stand number. This then updated the IRS.
I enjoyed flying the aircraft using raw data so, on the day in question, NAV was not displayed on the HSI on departure, just the basic VOR radial information. Alas the copilot had earlier entered EGPH (Edinburgh) instead of EGPF (Glasgow) when updating the IRS position. The required stand was not on the list so he accepted the airfield (EGPH) position as "good enough".
All was fine during the departure until well into the climb (using raw VOR data) I engaged the autopilot and reverted to the NAV display. No sign of the magenta line!! Changing the range revealed it well removed from our position. No problem, back to the VOR display!! Continued to Heathrow with the position slowly updating (DME/DME) and when we entered the Bovingdon hold all was back to normal.
Note... The 757 didn't have a position update on entering the active runway contained in the Lnav route which the 777 does (if I remember correctly) so there was no "position error flag" when comparing the actual position to that contained in the IRS brain.
What did it say on the wall of the briefing room wall at Hamble?
"Learn from the mistakes of others, you wont live long enough to make them all yourself"