At the point where Derek (actor/director Peter Jackson) falls from the cliff and lands upon the rocks below... You see a black bucket of fake blood get thrown out from behind the actual rocks.
I actually thought it added to the movie; it made it all that more laughable. One of the greatest movies ever made. Shot over 6 years on 8,000 dollars. Incredible. I don't know if it is a slip up, but Giles Copeland gets his hair cut a couple times in the movie.