This raises SystemStackError: stack level too deep

gistfile1.rb

require "rspec"
require "rspec/expectations"
require "rspec/mocks"

RSpec.configure do |config|
end

describe "SystemStackError: stack level too deep" do
  it "is raised by this" do
    file = "test_file"
    expected = "this should be written to test_file"
    File.should_receive( :open ).with( file, "w" )
    IO.any_instance.should_receive( :puts ).with( expected )
    File.open( file, "w" ) do |f|
      f.puts expected
    end
  end
end