Status Code 418 states that

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Any attempt to brew coffee with a teapot should result in the error code "418 I'm a teapot". The resulting entity body MAY be short and stout.

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See RFC2324 Section 2.3.2
The application of such a status code is boundless. Its utility, quite simply, is astonishingly unparalleled. It's a reminder that the underlying processes of computers are still made by humans. It'd be a real shame to see 418 go... Luckily, you all went out there and expressed your concerns regarding the ramifications of removing such a wonderful error code from the internet! We saved 418. The Internet Engineering Task Force is officially working on reserving 418. Consider its place in HTTP secured for a very, very long time. Thanks for everything Mark Nottingham, you put up a good fight! =)

FAQ

Below are excerpts from Matt Weinberger's Business Insider article.
> "What the heck is this 418 status code? Why is it important?"

If you've spent any time browsing the web, chances are pretty good you've run into a page with an error code on it. 

You've likely seen numbers 404 ("not found") or 403 ("forbidden").

Less commonly spotted is error code 418, which makes your browser proclaim "I'm a teapot."

If it sounds like a joke, it is: Way back on April Fool's Day in 1998, the Internet Engineering Task Force (IETF) — a group that sets internet standards — proposed "a protocol for controlling, monitoring, and diagnosing coffee pots." That document defined status 418 thusly: "Any attempt to brew coffee with a teapot should result in the error code '418 I'm a teapot.' The resulting entity body MAY be short and stout."

The error code has since become a running gag.

Go to Google.com/teapot, and see for yourself. Programming languages like Node.js and Google's Go both include the 418 error as a little Easter egg, as does Microsoft's ASP.NET framework. Someone even rigged a teapot to act as a web server, just so it can proudly display error 418 when you visit it.

On Thursday, however, the future of code 418 was briefly called into doubt. In a GitHub thread, Mark Nottingham, the chairman of the IETF working group that oversees hypertext transfer protocol (HTTP), argued that the 418 error was never a part of the standard, which governs how web browsers communicate with web servers.

> "Huh, ok. So why is this running joke important?"

"It’s a reminder that the underlying processes of computers are still made by humans," Brunswick said. "It'd be a real shame to see 418 go."

Others pointed out that the teapot status has been treated as a part of HTTP for so long that removing it could actually cause technical problems for many sites. That scored technical points for Brunswick's side.

> "What was the resolution?"

Ultimately, Nottingham and Brunswick came to an accord that seems to have made everybody happy. Nottingham filed a proposal to adopt 418 as an official HTTP code. If and when it's approved, "I'm a teapot" will officially become a core part of the web.

The issue is now closed, with programmers cheering that their teapots are safe. For his part, Nottingham is keeping a healthy sense of humor about the situation.  "If you ask me, it's a tempest in a ... ah, never mind," Nottingham wrote in an email to Business Insider.

Github Links: Node.js has ruled in our favor! Asp.net is keeping in 418! We won the Request scuffle! Go as well! (for the time being) #SAVE418 Endorsement from the author of HTCPCP-TEA: “I’m the author of RFC 7168, which extends HTCPCP to allow for the brewing of various teas in supporting Internet-connected teapots. I think you’re doing a great thing, trying to save something of the old whimsical Internet from the ravages of the Overly Professional.” About Me: I'm Shane Brunswick, an incoming high school sophomore. I'm a developer. I'm an avid fan of tea. Error code 418, an amalgamation of these passions, naturally means a lot to me. Some people want to expunge every trace of 418 from the world. I hope to stop them. Contact: save418movement@gmail.com