Answer: A Teapot
HTTP status codes are usually reserved for serious issues like server faults, missing files, and other matters of importance. Thanks to an April Fools’ Day joke in 1998, however, there’s a particularly curious (and entirely nonsensical) HTTP status code devoted to indicating the device contacted is, in fact, a teapot.
How did such an odd code come about? On April 1, 1998, the Internet Society released a tongue-in-cheek network protocol memo through the Internet Engineering Task Force’s Request For Comments (RFC) system. This memo, RFC 2324, entitled Hyper Text Coffee Pot Control Protocol, detailed the HTCPCP protocol, a derivative of HTTP intended for controlling, monitoring, and diagnosing internet-connected coffee pots.
Among the many codes included in the HTCPCP return/error code schema, there is a code, 418 (“I’m a teapot”), that is intended to politely inform the recipient that the device they are attempting to brew coffee with is, in fact, a teapot and incapable of producing the coffee they desire.
As you can imagine, the error code was never formally added to the HTTP error code schema and, should you actually receive the code, you’ve either encountered a webmaster with a sense of humor or an actual internet-enabled brewing machine coded by a tinkerer with a fondness for yesteryear’s April Fools’ Day pranks.