Had a pilot come to me complaining that the compass and ASI in his aircraft were in error.
I asked how he knew this, his reply "I have checked it against the shiney new GPS I bought at the weekend". It quickly became obvious that he did not understand the difference between what was displayed on the cockpit instruments and what was displayed on the GPS.
It was quite scary how many PPL were using GPS for navigation but have no understanding of what is displayed on the screen.