Over 7 years of flying long distances I've had several total (and thus obvious) GPS signal losses. These could have been caused by jamming or, in the last case which was today, by the GPS receiver being zapped by the multi-kilowatt radar as I taxied in front of it at a certain airport in the SE...
I have only once seen a subtle error, and that was using the now-discontinued Jeppesen Flitemap product (which is identical to the current Flitestar flight planning product but with GPS input, so you can run it as a GPS moving map) which showed my position a few miles off track, somewhere north of Crete. It was running on a tablet computer and was used for monitoring against a panel mounted GPS. However, this software is so bug-ridden anyway I would not like to rely on it too much. It would not suprise me that the unexpected and commercially nonsensical discontinuation of Flitemap (which suprised most Jepp users) was actually due to this issue. Now, their only moving map product is Flitedeck (which comes bundled with Jeppview 3) which I manage to crash every time I try to use it.
On the whole, GPS is exceedingly good. Problems are usually obvious, and (more importantly) there are few if any human configuration/operation errors which are not obvious. I am talking about a moving map unit with a decent screen, obviously; a GPS without a decent moving map is wide open to human error and that is even before humans start loading waypoints into them manually which is one thing you should have to do only in the most unusual cases (like the pos of your farm strip).