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

HTTP 100 (Continue) Status

A client can send a request message with a request body to determine if the origin server is willing to accept the request..

In some cases, it might either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking at the body.

Could be used to avoid traffic from rogue clients.. and/or where bandwidth is a precious commodity.

However, for full use of this feature there are some criteria for HTTP1.1 Client, Servers and Proxies. See the HTTP/1.1 RFC 2616 for further reading on HTTP Connections.