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.
About 4xx Client Error
The request has a problem the client needs to fix, like a bad URL, missing auth, or invalid input.