Here is my proposal for the 404 not found page.
The idea is based on famous misspell regarding Magento: Magneto. Everyone here has probably made the mistake typing too fast on the keyboard.
As we all know mistake typing an URL often leads to the 404 page.
Thus I would like to see a designed version of Magneto from X men in the 404 page.
Something like this:
If it was for a 503 page I also would have added the "You shall not pass" phrase as it's the same actor that plays Magneto and Gandalf in the movies. But I guess that would be irrelevant on a 404.
Regarding the captcha page I was thinking, what about a "are you human?" phrase with a picture of Phil Winkle's hair as he seems to have a lot of non human related accounts on the Internet. Maybe too much of a private joke here.