My favorite interview experience was when I passed a live coding exercise on Monday and had time to optimize, then completely failed the exact same problem on Friday with a different company because my nerves hit me.
I feel for you man, I drove 4 hours from Sacramento to Palo Alto for a great start up job. I nailed the phone interview and thought all was going to go well. The first onsite interview went pretty well. The second part went south and my nerves got me. I really wanted the job but I blew it. I got good feedback and I'm working on interview problems. It was my first interview in 20 years but again I really wanted the job.