306 (Unused)
The empty slot on the shelf: a number reserved for a code that never existed.
A 1996 draft proposed 306 Switch Proxy, a cousin of 305. The draft died before becoming a standard — but the number was already taken in early specifications, and taken it stayed. RFC 9110 lists it, drily: (Unused).
It is the strangest piece in the cabinet: never returned, never handled, meaning nothing. And yet it keeps its own shelf in the registry, so nobody reuses the number and confuses old clients. Every museum keeps one frame without a painting.
HTTP/1.1 306 (Unused)
(no server returns this line;
the number has been reserved, and empty, since 1996)