Padrino 0.11 以降で Better Errors を使う ref: http://qiita.com/tyabe/items/b5300572a9dfd7f7e039

app.rb

module Sample
  class App < Padrino::Application
    #...
    set :protect_from_csrf, except: %r{/__better_errors/\d+/\w+\z}

boot.rb

if Padrino.env == :development
  # Setup better_errors
  Padrino::Application.use BetterErrors::Middleware
  BetterErrors.application_root = PADRINO_ROOT
  BetterErrors::Middleware.allow_ip! ENV['TRUSTED_IP'] if ENV['TRUSTED_IP']
  Padrino::Application.set :exclude_from_protection, '/__better_errors' # Append this
  # ...
end

rack_protection.rb

if Padrino.env == :development
  module Rack
    module Protection
      class AuthenticityToken
        def accepts_with_allowed_path?(env)
          if defined?(Padrino::Application.exclude_from_protection)
            [Padrino::Application.exclude_from_protection].flatten.each do |path|
              return true if Regexp.new(path).match(env['REQUEST_PATH'])
            end
          end
          accepts_without_allowed_path?(env)
        end
        alias_method_chain :accepts?, :allowed_path
      end
    end
  end
end