[meowface]

It depends how you define validate. If the HTTP headers are malformed, sure. If a JSON or XML API receives malformed JSON or XML, sure. But if it's "register this account" and the username already exists, or "pay with this credit card" but it's not a valid credit card number, or "process this invoice" but the total is missing, then I think it should be a 200 status with an error message. It's validating the request was received and loaded by the application vs. validating the business logic requirements for the request's data payload.

Obviously if you send a TAIL method request, you should get a 405, and if you send Accept: eggs/*, you should get a 406. If a route doesn't exist, you should get a 404. If you fail HTTP basic auth, you should get a 403 (but why are you using HTTP basic auth?). If you want certain paths to never be accessed for some reason, you should return a 401.