418 I'm a teapot

Client error: the problem lies with the request. Repeating it unchanged will normally fail again.

When a server should send it. Never as a real error. The number is reserved because many servers return it as a joke from RFC 2324, the April Fools' Hyper Text Coffee Pot Control Protocol. Some sites use it to turn away bots.

Common causes. Some sites and APIs return it on purpose to turn away bots.

What to do. Nothing, unless a service uses it to block you; then treat it like 403.

Browsers and HTTP clients. Clients treat it as an ordinary 4xx client error.

Google Search. Like every 4xx except 429, it tells Google the content does not exist: an indexed URL is removed from the index, and a new one is not processed. Crawl frequency for the URL gradually drops. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Not cacheable by default (RFC 9110 section 15.1 does not list it). A cache stores it only when the response says so with Cache-Control or Expires.

HTTP/1.1 418 I'm a teapot
Content-Type: text/plain; charset=utf-8

I'm a teapot

location /example {
    return 418;
}

RewriteEngine On
RewriteRule ^example$ - [R=418,L]

http_response_code(418);
echo 'I\'m a teapot';
exit;

res.writeHead(418, { 'Content-Type': 'text/plain' });
res.end('I\'m a teapot');

# in a view function
return 'I\'m a teapot', 418