The <title> of 404 pages reads:

  Error 404 (Not Found)!!1

I find it peculiar that a company as large and as corporate as Google still has this level of playfulness in it. As a long-time Linux user, it left me puzzled for many years until I was forced to use a Mac at work. My hunch is that because the title element is not surfaced to the user on a stock Mac in any meaningful way, this <title> managed to survive because nobody with decision-making power will ever see it.