Well, ok, it’s a joke status code, from an April 1st joke about the Hyper Text Coffee Pot Control Protocol. It’s a possible error code, if the thing you’re asking to make you coffee is, in fact, a teapot, and thus cannot possibly make you coffee. From the RFC itself:
2.3.2 418 I’m a teapot
Any attempt to brew coffee with a teapot should result in the error code “418 I’m a teapot”. The resulting entity body MAY be short and stout.