ActiveSupport::CoreExtensions::Array::Conversions#to_xml modifies the passed option hash

test = {}
[].to_xml( test )

Now test is equal to the following instead of {}:

{:indent=>2, :builder=><?xml version="1.0" encoding="UTF-8"?>
<nil-classes type="array"/>
<inspect/>
}

The situation that makes this bad is:

class SomeController < ApplicationController
  TO_XML_OPTIONS_HASH = { :indent => 3 }







  def some_action

render :xml => SomeModel.all.to_xml( TO_XML_OPTIONS_HASH )

  end
end

When class caching is on, this causes the TO_XML_OPTIONS_HASH to be modified and the builder in the hash will get appended to every time the action is called.