Not GPS Error, but human error.
I've used a GPS for years - my first was a garmin black and white model. In that time, I've only once been badly misdirected; it was my fault, and I didn't follow it, so no harm was done.
I'd been in Glasgow for a few days, and was driving back to Peterboroough. On approaching a roundabout, it directed me the wrong way around it. Of course, I ignored the directions. It then appeared to randomly direct me around some roundabouts the right way, and some the wrong way.
When I finally hit the motorway, it immediately directed me back off again. Ignoring that, it then directed me off the next junction, and the next. Thinking something was wrong, I switched off navigation, and for the rest of the journey just used the GPS to tell me where I was.
A couple of days later, I decided to take a look at my "broken" GPS, and soon found the mistake. While in Glasgow, I had used it in pedestrian mode, and had forgotton to switch it back. So it tried to direct me the quickest way (to walk) around a roundabout, and tried to guide "the pedestrian" off the motorway.
Fortunately, I've long had the attitude that I treat my GPS just like a passenger with a map. I listen to what they say, but then decide if what they are telling me to do is safe and appropriate.