[Curmudgeon]

There are a few odds and sods based in reality that I've thrown out from time to time that throw a spanner into the usual GURPS defaults for my players.

1) Any orienteering above 60°N (or below 60°S) cannot make use of a magnetic compass, but can resort to using an astral compass with ephemeris tables (essentially a land-going sextant with astronomy tables). This switches them from Orienteering to Navigation.

2) In pre-1970 settings, many maps of the north at 1:250,000 scale are blank sheets of paper with a grid system superimposed, i.e. they are featureless. Post-1950, they can make use of inertial navigation systems which give distance and direction from the starting point but require an accurately known starting point to know exactly what your current position is.

3) There is one supposed real-life navigation critical failure that I did manage to use in one game. I'll use the original anecdote though:
Naval midshipman (on a ship in the Pacific theatre): Here are my calculations for our position, sir.
Captain (after perusing the calculations): Young man, kindly remove your headdress.
Midshipman: Sir?
Captain: Young man, we are on sacred soil. By your calculations, we're in the middle of Westminster Abbey.