It's been 20 years today (Oct 16, UTC). Hard to believe.

[kmedcalf at dessus.com (Keith Medcalf)]

>For example just because they sent you a seemingly malformed HTTP
>request, and given that 4xx is for error codes, doesn't mean you
>should return "420 You must be high!" and expect to be understood.

Actually, you can, and the sender of the request MUST understand.  
The relevant part of the applicable RFC says:

   HTTP status codes are extensible.  HTTP clients are not required to
   understand the meaning of all registered status codes, though such
   understanding is obviously desirable.  However, a client MUST
   understand the class of any status code, as indicated by the first
   digit, and treat an unrecognized status code as being equivalent to
   the x00 status code of that class, with the exception that a
   recipient MUST NOT cache a response with an unrecognized status code.

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The fact that there's a Highway to Hell but only a Stairway to Heaven says a lot about anticipated traffic volume.