404.html.haml

%p Whoops! The resource you were looking for wasn't found.

application_controller.rb

class ApplicationController < ActionController::Base
  def render_404
    render :template => "errors/404", :status => "404"
  end

  def rescue_action_in_public(exception)
    case exception
      when ActiveRecord::RecordNotFound
      when ActiveRecord::RecordInvalid
      when ActionController::RoutingError
      when ActionController::UnknownController
      when ActionController::UnknownAction
      when ActionController::MethodNotAllowed
        render_404
    end
  end
end