fail.rb

class MockController
  def url_for(options)
    super(options)
  end
end

module ActualUrlFor
  def url_for(options)
    options.values.join("/")
  end
end

klass = MockController.clone
klass.class_eval { include ActualUrlFor }
p klass.new.url_for(:controller => "omg", :action => "omgomg")

# testing.rb:3:in `url_for': super: no superclass method `new' for #<#<Class:0x00000100865000>:0x00000100864f10> (NoMethodError)
#	from testing.rb:15:in `<main>'

fail2.rb

class MockController
  def url_for(options)
    super(options)
  end
end

module ActualUrlFor
  def url_for(options)
    options.values.join("/")
  end
end

klass = MockController.clone
klass.class_eval { include ActualUrlFor }
obj = klass.new
obj.to_s
p obj.url_for(:controller => "omg", :action => "omgomg")

# testing.rb:3:in `url_for': super: no superclass method `to_s' for #<#<Class:0x00000100864e70>:0x00000100864d80> (NoMethodError)
#	from testing.rb:17:in `<main>'