We should be able to look at see if the child has a data-react-id attribute but isn't rendered by this copy of React and consequently warn.

See https://groups.google.com/forum/#!topic/reactjs/mdwfKKJ0L2Q.

return ReactMount._renderSubtreeIntoContainer(
      parentComponent,
      nextElement,
      container,
      callback
    );
  },
  _renderSubtreeIntoContainer: function(parentComponent, nextElement, container, callback) {
    ReactUpdateQueue.validateCallback(callback, 'ReactDOM.render');
    invariant(
      React.isValidElement(nextElement),
      'ReactDOM.render(): Invalid component element.%s',
      (
        typeof nextElement === 'string' ?
          ' Instead of passing a string like \'div\', pass ' +
          'React.createElement(\'div\') or <div />.' :
        typeof nextElement === 'function' ?
          ' Instead of passing a class like Foo, pass ' +
          'React.createElement(Foo) or <Foo />.' :
        
        nextElement != null && nextElement.props !== undefined ?
          ' This may be caused by unintentionally loading two independent ' +
          'copies of React.' :
          ''
      )
    );
    warning(
      !container || !container.tagName ||
      container.tagName.toUpperCase() !== 'BODY',
      'render(): Rendering components directly into document.body is ' +