One year later........... I had a strange experience last Saturday. I was sailing in Casco Bay (Maine, lots of rocks and ledge), running up close to two small rocky islands, and deciding whether to run between them (narrow, shallow) or around the outside of the outermost one (deeper, lots of sea room).
I looked at my Garmin and it showed my boat already outside the outer island, quite a distance from my actual position - it was a clear evening with plenty of light, sun just setting, full moon about to rise - good visibility. I checked several times, the Garmin - normally extremely accurate - was showing the completely wrong position for my boat.
Since I was checking for depths, I could have gone very wrong if I wasn't relying on a paper chart and visuals, with the Garmin as a helper. When I rounded the outer island and tacked into it's lee and onto my anchor, the position was shown correctly.
So while the chart on the Garmin was accurate, my position as shown was totally inaccurate for a short while.
Glad it wasn't foggy!