I guess it's for indication of when you've typed something wrong or accidently clicked a bad internal link. I like the animation on it, very flashy. I wouldn't say it's a easter egg though, it's more like rare.
@Helios: that was my intention... well flickering neon signs actually. I dislike that the flickering is not random but I didn't want to use flash or something like that... probably good enough for a simple 404 page though.
@Qrchack: cs3 - doesn't really matter. you can do something primitive like that with much older versions.