Any further finds?
https://worldofspectrum.org/nosuchpage
https://www.masswerk.at/nosuchpage
(via Christoph S)
I wouldn’t really call mine (masswerk.at) a retro computing page, other for using the georgeous art by Peter v. Tresckow (for IBM Germany, 1976). There was a previous one, which was more retro styled, but its status eventually changed from 404 to 451 (“Unavailable for Legal Reasons”, i.e. cease and desist letter by Taito – as it turns out, the sprites are trademarked). Some images are still available here.
I really should come up with a proper retro computing themed one.
Anyway, thanks for mentioning! 
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“The message that the requested resource cannot be found cannot be found for legal reasons” (suggested status code, 455 – “Error not allowed”). 
Regarding the Space Invaders one, the original one worked, because the error code came inbetween the user and the site-operated invaders as an obstacle, very much reflecting the situation the user found herself while interacting with the site. Using the company name for this may not be that wise…
Regarding 455, this was meant as a combination (OR) of 405 and 451.
This is rather ingenious! (And it works for the entitire 401-405 range as well!)
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BTW, the original Space Invaders error page is still available behind a login:
https://www.masswerk.at/status/1/?404&theme=0
(user is what you would encounter in space, especially in this game, password is the path as in “/status/1/” without any slashes)
As may be guessed, the first parameter (or the bare query) is the status code, the second one a color scheme (also to be selected by a tiny widget at the bottom of the page). E.g., if you feel a bit like James Bond, try https://www.masswerk.at/status/1/?007&theme=4
This is a kind of indirect retro, but the popular Varnish HTTP accelerator/proxy software uses HTTP “503 Guru Meditation Error” for its internal errors: Guru Meditation - Wikipedia
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