I didn't mean to say that (but I guess it looks like I did). Obviously there are other ways if singnalling receipt of a bad message (throwing exceptions for instance). METHOD_MISSING seems more natural to me at least.

What I meant to say is that in REST, the set of messages to which an object will respond is a function of the object's current state. With SOAP, a URL endpoint always takes the same set of messages (until the code is rewritten or something).