A friend of mine has just returned from a National Mark laying course in preparation for the Olympics and has found out that his fairly expensive Marine GPS was positioning himself 300m differently to the Instructor /Examiner on several occasions. When he returned home and phoned Garmin they weren't exactly helpful and thought that 300m was an acceptable degree of inaccuracy. Martin then explained that the competitors at the Olympics may not have the same view. Garmin could only suggest that a more expensive GPS would probably be more accurate.
So I guess you can't believe everything your GPS is telling you as it sounds that they aren't as accurate as Garmin would like to make you think.