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.

ReactUpdates.batchedUpdates(
      batchedMountComponentIntoNode,
      componentInstance,
      container,
      shouldReuseMarkup,
      context
    );
    var wrapperID = componentInstance._instance.rootID;
    instancesByReactRootID[wrapperID] = componentInstance;
    return componentInstance;
  },
  /**
   * Renders a React component into the DOM in the supplied `container`.
   *
   * If the React component was previously rendered into `container`, this will
   * perform an update on it and only mutate the DOM as necessary to reflect the
   * latest React component.
   *
   * @param {ReactComponent} parentComponent The conceptual parent of this render tree.
   * @param {ReactElement} nextElement Component element to render.
   * @param {DOMElement} container DOM element to render into.
   * @param {?function} callback function triggered on completion
   * @return {ReactComponent} Component instance rendered in `container`.
   */
  renderSubtreeIntoContainer: function(parentComponent, nextElement, container, callback) {
    invariant(
      parentComponent != null && ReactInstanceMap.has(parentComponent),
      'parentComponent must be a valid React Component'
    );