407 Proxy Authentication Required
The middleman's 401: it is the proxy demanding credentials, not the site.
It is the mirror of 401, one step earlier: the one demanding credentials is not the destination site but the proxy along the way. The challenge comes in Proxy-Authenticate; the client's answer, in Proxy-Authorization.
Its habitat is the corporate network with a mandatory proxy. curl failing with 407 Proxy Authentication Required is a rite of passage for anyone debuting behind a big company's firewall.
HTTP/1.1 407 Proxy Authentication Required
Proxy-Authenticate: Basic realm="proxy-corporativo"
Content-Length: 0