Of course as most people also know numerous things can prevent a compass from pointing to Magnetic North. Large deposits of nickel or iron can throw off the accuracy of a compass. Even a small amount of iron or nickel held near the compass can give a false reading.
If the compass isn't held level or isn't allowed to pivot freely it may give false readings, also. Some believe that even atmospheric conditions such as a lightning storm could cause a compass to give false readings. This is why everyone from pilots to amatuer orienteering enthusiasts do not rely soley on compasses to get from point "A" to point "B".