418 is a class 4xx error, which means the client erred and shouldn't repeat the request as-is. That is, any unrecognized error beginning with a 4 must be handled as if it were the base error class, 400, "Bad Request".

> the server cannot or will not process the request due to something that is perceived to be a client error (for example, malformed request syntax, invalid request message framing, or deceptive request routing).