I also really like the one with earphones in the glass of water. Very "there is an error+music" related, I also like the ones with soundwaves making error codes. but I'd save these for the one or two where a meta-joke like the above doesn't make sense or for situations where the error happens inside a frame or in a section of page where you'd only need a small image instead of a full page.
I also like the design of the error code in https://tickets.metabrainz.org/secure/attachment/12659/idea6-500.jpg that sort of "things are coming apart at the seams" I'd use this paled out as a background maybe?
Further I suggest we put this to the community to suggest ideas for other status codes, esp meta jokes re the error code description and relevancy to musical themes - There are many error codes with humorous potential: 300 Multiple Choices, 301 Moved Permanently, 403 Forbidden, 418 I'm a teapot (:D), 502 Bad Gateway and so on (full list: https://en.wikipedia.org/wiki/List_of_HTTP_status_codes)
As for BookBrainz, there are equally many ideas from books, a book with missing pages - censored out text on a page, etcetera.
as reo mentioned in the chat. we definitely need one with a cat (and also one with a teapot)
check out https://beta.musicbrainz.org/release/4914a9d4-aac0-4bb9-9cda-be0e6ff90db7
this is music based on status codes, they are https://creativecommons.org/licenses/by/3.0/ and the music is fairly good? https://thingsinjars.bandcamp.com/album/http-status-codes we could put an embeddable player on the appropriate page ( https://thingsinjars.bandcamp.com/track/404-not-found for 404 for example)
Great idea for ListenBrainz maybe?