Stupid reason imo. Adding childish humor to a protocol seems like harmless fun until you remember how much of a mess network protocols, and their specs, and their implementation is. Maybe you think a 418 error page is funny, but I think you’ll appreciate it a lot less when your application gets a rather confounding error because some service provider wanted to remind you that the underlying processes of computers are still made by humans, rather than implement the actually correct response.
I promise that knowing it’s supposed to be a joke is _not_ going to make you less frustrated if you ever end up having to debug a non-descriptive 418 error. To make it worse the description and the error code aren’t even consistent. 4xx is a client error, but your web server being a teapot is definitely a server side problem.
No, it's a great canary in the coal mine. Anyone who doesn't implement proper handling of 418 gets their general error handling facilities tested (4xx).
