Did it happen just once, or it kept repeating itself over and over? How far did it move from your actual position? (How far was the river from the road for example?) When the problem occurred did the receiver have clear view of the sky in all directions or was the road among trees or buildings? Where is the GPS receiver placed? On the roof of the car or inside the car? If inside, then where exactly?
It is possible you have a faulty receiver. But if it only happened once I wouldn't worry about it (or if the distance was within 10 meters/30 feet.)