I'm currently helping with an art & writing collaboration website called Scribblehack, and was asked to do the 404 error page graphics. The theme of the website is exploration and adventure, so I decided to make the numbers 404 large, to act as windows into other worlds. I hoped this would make it seem as if Scribblehack was a vast universe, and that this page gave glimpses of places that were still unexplored.
I also wanted to integrate the main graphic with the banner, and the banner with the top navigation bar, so pipes connect the numbers to the banner, and the sand at the top of the banner breaks up the straight line of the navbar a bit.
The pipes remind me a bit of laboratories, which I hoped would play to visitors' senses of curiosity (curiosity and exploration go hand in hand, right?). The lab reference helped me to have a bit of fun with the error message, too.