Blackboard goof continuity

Luke Evans is talking to the group infront of a blackboard containing and map/plan. Someone jokes and calls him a school teacher, he turns and starts rubbing chalk out but in final shot the blackboard is still neatly covered in untouched text. How can the BBC be so sloppy? Great drama however I am bitterly disappointed with the low standards of production. How can this slip through the net?? Disgusting lol!!!