418 I'm a teapot
The IETF has a tradition to publish one or more april-fools RFC documents
every year. Among the most famous are IP over Avian Carriers with Quality of Service,
and of course Hyper Text Coffee Pot Control Protocol, which
introduced the 418 I'm a teapot HTTP status.
This status-code is of course not an official one. You won’t find it in the IANA HTTP Status code registry. However, people have such a liking to it that it’s supported in many HTTP libraries.
About a year ago, the HTTP working group tried to reach out to
various projects to try and remove their support for 418, so the number
could be re-used for a new purpose, but this was met with a lot of resistance,
even spawning https://save418.com.
Several implementations of HTCPCP can be found, including:
The 418 code also appears as easter eggs in numerous systems.
Wikipedia has a page on all the IETF april fools jokes, which is well worth a read. It also features the HTCPCP-TEA extension!
All we need now is a WHATWG living standard that just solidifies everyone’s buggy implementations (looking at you URL standard), and a W3C committee that forks it once per year.