I agree with chris below - absolutely no redirection!
Surely that's easy to implement on current routes_onerror?
Yeah, this feature is really necessary if you want to be successful in some even trivial tasks, not to mention SEO... as a matter of fact this should be done frome the very first.
You should be able to define your own controller and view for error pages and they should return the proper status codes (404, 400, 401, etc.) without redirection.