While this is true, it's also not a huge problem, exactly:
> HTTP status codes are extensible. HTTP applications are not required
> to understand the meaning of all registered status codes, though such
> understanding is obviously desirable. However, applications MUST
> understand the class of any status code, as indicated by the first
> digit, and treat any unrecognized response as being equivalent to the
> x00 status code of that class, with the exception that an
> unrecognized response MUST NOT be cached. For example, if an
> unrecognized status code of 431 is received by the client, it can
> safely assume that there was something wrong with its request and
> treat the response as if it had received a 400 status code. In such
> cases, user agents SHOULD present to the user the entity returned
> with the response, since that entity is likely to include human-
> readable information which will explain the unusual status.
This text was basically unchanged in httpbis: http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-2...