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

Status codes :

• When URI http://www.domain.invalid/index.php?id=44 is called, if the query (id=44) couldn't return ressource, why not return a status code 404?
• When URI http://www.domain.invalid/index.php?id=foo is called whereas id only accepts integers, why not return a status code 400?
• Why, when you enter the wrong login/password, almost all web application return a message like "Authentication failed" with status code 200 (ok, no problem, you do it well) instade of 401?

Yes, status codes seems to be a kind of secret functionality of HTTP to some web developers... But I wonder if the most occult of all the "features" of this protocol isn't its RFC!