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

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.