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
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:
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.