506 Variant Also Negotiates
The negotiation that points at itself: a misconfiguration with its own code.
RFC 2295, experimental, from 1998, proposed transparent content negotiation: the server publishes the list of variants and the client chooses. 506 signals the internal accident: the chosen variant is negotiable too — a loop of mirrors in the configuration.
Since transparent negotiation never took off, 506 is doubly rare: the misconfiguration of a mechanism almost nobody turns on. It remains in the IANA registry, marked experimental since the last century.
HTTP/1.1 506 Variant Also Negotiates
Content-Type: text/plain
variant list points at a negotiable resource