 Before the final battle, Arthurs knights are camped around a  "Stonehenge" like formation on a hill. The rocks were fake and part  of the set. According to John Boorman (qv), some American tourists  were driving by down below and saw the formation. Thinking they were  real, the tourists hiked up the hill, and Boorman had to explain to  them that they were not real but part of a movie set.