davidbisset, 
How to make an ineffective 404 page.
https://ericwbailey.website/published/how-to-make-an-ineffective-404-page/
(mostly) a case against blindly trying to be "cute" with 404 error pages, but goes into more past just design.
"The other thing is 404s can still be quirky!... Just make sure that their immediate needs are met before trying to get creative."