You see, I think you are right. Whenever I see a discussion about GPS reliability it is actually about the accuracy of the maps, not the GPS absolute position information.
However we have seen remarks like 'there was a problem over ORTAC' which its is not clear whether it is a mapping or navigational fault.
GPS does have errors eg signal bending which could not be detected at the minumum number of satellites. Also you could have 5 satellites all from the same unfavourable direction and thus having multiple atmospheric distortion errors not correctable by averaging.
I can't think of anything else ie if you are doing a VOR-style overlay approach (waypoint coordinates validated) with 7 satellites the GPS will either show nothing or the correct track and distance.
Anybody experienced anything different?