421 Misdirected Request
Right door, wrong house: this connection does not serve that domain.
HTTP/2 reuses connections across domains that share a certificate and an address. Sometimes the request lands on a server that answers the IP but is not authoritative for that Host — and 421 warns: retry over another connection.
It was born with HTTP/2, in RFC 7540, and was promoted to general HTTP in RFC 9110. Clients handle it on their own — they reopen the connection and repeat the request — so users never see it: a backstage code, from the era when one connection serves many houses.
HTTP/1.1 421 Misdirected Request
Content-Length: 0
Connection: close