Mr Pratchett parodied Macbeth there -
"The stone was about the same height as a tall man, and made of bluish tinted rock. It was considered intensely magical because, although there was only one of it, no-one had ever been able to count if, if it saw anyone looking at it speculatively, it shuffled behind them. It was the most self-effacing monolith ever discovered."

(I'm not keen on him. Not because of his writing but because I went to a school play in Andover once and he sat in the middle of the audience with his stupid hat on the whole time, the prat).