my magnetic field sensor is quite unstable under several circumstances. I don't think this is a hardware issue so it can be reproduced. It works fine in home but it loses its accuracy ouside:
a) inside metal objects (i.e. cars and buses) - as you rotate the phone the N sign moves unpredictably. Try to notice direction of north, then rotate phone by 90 or 180 degrees. There will be significant difference in measured change and real movement.
b) near strong electromagnetic field - (i.e. electric lines) - N can drift off of its original place
c) inside some vehicles lags in measurements can also be noticed.
Basically you can go to the center of a city and look at any compass app. You should be able to reproduce it.