GPS have errors?
I was stunned the first time my GPS brought me, with authority, 100km from my intended destination. They are usually accurate enough that we don't tend to doubt them. Beside their instructions are hard to review, you know your trip once you are in the car on your journey, with all your planning already done and without access to alternative sources. Afterall, you ditched the map so you didn't need to pack luggage and have to fuss with the lost and found dept of your least hated airline.
My old TomTom has done this to me several times. So much so that now I carry both paper printout of google maps and the GPS plus now the Android phone. I guess we are getting our money's worth when you pay the equivalent of maybe $20 for the entire maps of planet earth.