Returns a mock object which will detect calls to any instance of this class.

  Product.any_instance.stubs(:save).returns(false)
  product_1 = Product.new
  assert_equal false, product_1.save
  product_2 = Product.new
  assert_equal false, product_2.save

[Source]

     # File lib/mocha/object.rb, line 122
122:   def any_instance
123:     @any_instance ||= AnyInstance.new(self)
124:   end