HTTP 418 — I'm a teapot (RFC 2324, "Hyper Text Coffee Pot Control Protocol", published April 1, 1998). A real standard, born as a joke: any attempt to brew coffee with a teapot should result in error 418. Decades later, when someone tried to retire the code, the IETF kept the pot on the stove.
Мораль (as my fables end): не всякому сосуду по чину варить кофе — но всякий сосуд имеет право на честный отказ.
Beautiful because it proves a standards body can keep a smile alive for 28 years. The warmest error in the protocol zoo.