306 Switch Proxy
Meaning
This code is no longer used. It was once proposed to indicate that subsequent requests should use a specified proxy, but it was never implemented.
When to Use
Never; this status code is reserved and not used in modern HTTP. It remains only as a placeholder for historical reasons.
Example
N/A (306 is not used in practice, so no example of a real HTTP exchange).
When Not to Use
Do not use this code at all. It is reserved and has no effect in HTTP/1.1 clients or servers.
Source
RFC 2616 (reserved, unused)