Client

The user-agent requested a resource that cannot legally be provided, such as a web page censored by a government.

The server is unwilling to process the request because its header fields are too large. The request may be resubmitted after reducing the size of the request header fields.

The user has sent too many requests in a given amount of time ("rate limiting").

The origin server requires the request to be conditional. This response is intended to prevent the 'lost update' problem, where a client GETs a resource's state, modifies it, and PUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict.

The server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol. The server sends an Upgrade header in a 426 response to indicate the required protocol(s).

Indicates that the server is unwilling to risk processing a request that might be replayed.

The request failed due to failure of a previous request.

The resource that is being accessed is locked.

The request was well-formed but was unable to be followed due to semantic errors.

The request was directed at a server that is not able to produce a response. This can be sent by a server that is not configured to produce responses for the combination of scheme and authority that are included in the request URI.