506 Variant Also Negotiates
Meaning
The server has an internal configuration error: the chosen variant resource is configured to engage in content negotiation itself, leading to a circular reference and no appropriate variant.
When to Use
When a server is using transparent content negotiation and a configuration problem results in a negotiation cycle (for example, the resource is configured to reference itself as a negotiation variant). This is a rare error mainly described in negotiation mechanisms.
Example
A server configured for content negotiation ends up with a variant that also tries to negotiate. When the client requests a resource, the server finds that the variant is not a concrete resource but another negotiable resource, so it returns 506 Variant Also Negotiates indicating a server configuration issue.
When Not to Use
Do not use for normal content negotiation failures (those might result in 300 or 406). 506 specifically signals a server configuration issue in the negotiation setup. It is very rarely encountered in practice.
Source
RFC 2295