The worst example of compass error that I've come across led to the Wheal Owles disaster in 1893 which led to the death of 20 miners. This was due to magnetic variations not being taken into account over several decades worth of surveying which led the company to believe that they were working on two separate lodes (Drea and Cargodna) which were in fact one and the same. The consequences of this mistake was that abandoned flooded workings were holed into when operations were conducted in that direction. The bodies were never recovered.