As has been recently advertised (in here, here, here also), a recent British Airways flight from London City Airport destined for Düsseldorf mistakenly flew its passengers to Edinburgh instead.
I'm a layman in terms of aviation, but I (and I'm sure many more do) do see aviation as a strictly regulated area full of double-checks, double instruments (even 2 pilots!) to make sure an airplane is doing exactly what it's supposed to be doing.
Perhaps mine is a rather car-centric view, but I can't imagine starting the engine of the car without knowing clearly where I'm heading, which landmarks, roads (and so on) I'm supposed to pass by during the route.
TL;DR:
Must (or should) both pilots know where he airplane they are flying is going to? Or do they simply follow an approved flight path?