Ashcroft,
I cannot pinpoint exactly where I fell down. I was able to answer all of the technical questions however either I wasn't confident enough in my reply or wasn't completely correct. I fumbled a bit on a question about monsoons and I think because the interviewer could see I was having a bit of a problem kept on that subject, letting me dig my own grave. As Mongok cowboy says at the top of the page you really have to go in no less prepared than 100%. It was my first interview in about six years and maybe I just didn't fully appreciate the intensity and levels of study required.
I hope it hasn't put you off but maybe get a friend or two (one asking HR and one tech q's) to grill you on various subjects before you go for your interview.