In some instances where the GPS waypoints show up 30-100 yards off on the map, it is the map that is wrong. No one could tell until the GPS came along. This summer I was in an area of WCPP where this happened. The trail I marked would show up starting 100 meters out in the water when viewed on the map screen.

This is why a GPS option is to set it on "lock on road." It corrects and puts you on the road and not off to the side. In actual use on the trail, if you are using the same datum as the person who marked the waypoint it should put you really close.