300 Multiple Choices
There is more than one version; you choose — says the server, and nobody chooses.
It is agent-driven content negotiation: instead of picking the language or format on its own, the server returns 300 with a list of alternatives in the body — and may point at the preferred one in Location.
No standard format for that list ever caught on, and no browser grew an interface for choosing. What remains is Apache, which returns it when a type map turns ambiguous — nearly the only place this specimen still breathes.
HTTP/1.1 300 Multiple Choices
Location: /manifesto.pt.html
Content-Type: text/html
Vary: negotiate, accept-language