Hidden features of HTTP

Question

What hidden features of HTTP do you think are worth mentioning?

By hidden features I mean features that already are part of the standard but widely rath

Answer 1

It's got to be the 418 I'm a teapot status code, part of the Hyper Text Coffee Pot Control Protocol (an extension to HTTP). Makes me laugh every time.

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.