418
4xx Client ErrorI'm a Teapot
An April Fools' joke from 1998. The server refuses to brew coffee.
418 I'm a Teapot comes from the Hyper Text Coffee Pot Control Protocol, an April Fools' RFC from 1998. It's not a real error you'll hit in normal use, but it survives as an Easter egg and some APIs return it as a playful 'no' or to mark a blocked request.
What 418 looks like on the wire
BREW /pot-1 HTTP/1.1 Host: teapot.example.com Content-Type: message/coffeepot HTTP/1.1 418 I'm a Teapot
About 4xx Client Error
The request has a problem the client needs to fix, like a bad URL, missing auth, or invalid input.
Where it's defined
418 I'm a Teapot is defined in RFC 2324 §2.3.2. MDN's 418 reference covers browser behavior and examples.