Jasonw points out a great site called moviemistakes.com that covers the gooks and fun mistakes made in movies. Kind of interesting for a geek like me.

When filming the battle of Helm's Deep the makers used a computer programme called Massive, where each person has its own "mind", making it far easier to generate huge battles, because each person can be allocated a "side", and will then react/fight accordingly, in a variety of different styles (depending on the circumstances they find themselves in), rather than having to create/program each army member individually. The first time they used this for the battle of Helm's Deep, the defenders of Helm's Deep ran away.