You see, a basic GPS receiver simply has to figure out its position and put that into some user interface. In the old days, that was as primative as displaying the lat/long numbers and elevation number. Little by little through the years, users have demanded more mapping features. It has gotten to the point now where many users get all caught up in seeing their position as displayed on some mapping database, and they are willing to pay more and more for mapping data that they may or may not understand or be able to fully utilize. Some users have gotten so dependent with their assumption that the instrument has some kind of map database, so it must be accurate. That gets a few of them into trouble.
There is this nasty little secret in the way a typical GPS receiver works improperly once in a while. The receiver does what we call a blunder. A blunder on the receiver can make you appear to be thirty miles away and traveling at 400 miles per hour, for example. Now, the logical human would look at that and think, "No way!" He shuts it down, counts to ten, and then starts it up again. By then, the weird but real situation that had set up is now gone, so the position will be fairly real now.
I've seen blunders happen in normal receivers and in big base station receivers. If your whole navigation is based on that receiver, you might be S.O.L.
Personally, I plan my route on an electronic topo map, then color print it, stick it in a plastic bag, and tuck it in my pocket when I am out on the trail. I'll pull out the GPS receiver and turn it on only when or if necessary, maybe once or twice per day.