Hidden features of HTTP

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.

The fact that referrer was misspelled and it was decided that the misspelling should be kept.

Obvious answer: PUT, DELETE, TRACE, OPTIONS, CONNECT methods

Most people know about the GET and POST methods because that's what they use when building forms. Browsers also use HEAD a lot. The other methods are much less well-known; they are mostly used by more specific applications.