I can cap that.
Met Office JHQ RAFG's first attempt at using magnetic smarties RED AMB YLO GRN WHT BLU on a map of NW Europe failed to note that the back-plate for the large wall map was non-ferrous. Just before my time there.
The C Met O [whose invention it was] was apparently displeased. The staff delighted.
It took a junior assistant to diagnose the problem, it was said.
Thereafter the board worked well, although labour intensive, bridging the gap to the computer age.