Those “mobile” GPS like a Garmin 496 have a internal Lithium battery and most times a 12V plug from the airplane that keeps it charged. So you have two energy sources and the battery alone will normally last longer than your fuel. Sure you have the maps and you better mark some positions with time, but if the weather goes south and there is some additional distraction it is easy to get your position wrong. There is a reason that in the USA you still can find mountains marked with large white numbers and airports names in large letters on the roof.
Ask how many pilots with some VFR single pilot time got at some time in their cariere their position wrong at least once.
Normaly no big deal as long as you do CCC. (communicate confess comply)