I have experienced bad locations from GPS, but not so bad that I didn't know where I was. In a fairly deep canyon (and probably less than optimum satellite geometry), the GPS told me I was in the bottom of the canyon, while I, eagle eyed archaeologist that I am, could readily see that I was well up the canyon slope at an obvious bend in the canyon.

You must think when you are navigating. Compasses and maps have their imperfections, as well. Don't trust any tool mindlessly.