RE: Lesson Learned the Hard Way: Compass
GPS have occasional hiccups, but i bet you werent getting good enough signal from enough satellites. There are about 25 satellites in the sky that GPS's will use to triangulate. The GPS needs atleast 3 signals to triangulate correctly. My guess is you were surronded by something that was disturbing the signal, so it was getting confused.
Take the GPS into a more open area and use it again to see how it works.