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.

delete instancesByReactRootID[prevComponent._instance.rootID];
    ReactUpdates.batchedUpdates(
      unmountComponentFromNode,
      prevComponent,
      container,
      false
    );
    return true;
  },
  _mountImageIntoNode: function(
    markup,
    container,
    instance,
    shouldReuseMarkup,
    transaction
  ) {
    invariant(
      container && (
        container.nodeType === ELEMENT_NODE_TYPE ||
        container.nodeType === DOC_NODE_TYPE ||
        container.nodeType === DOCUMENT_FRAGMENT_NODE_TYPE
      ),
      'mountComponentIntoNode(...): Target container is not valid.'
    );
    if (shouldReuseMarkup) {
      var rootElement = getReactRootElementInContainer(container);
      if (ReactMarkupChecksum.canReuseMarkup(markup, rootElement)) {
        ReactDOMComponentTree.precacheNode(instance, rootElement);
        return;
      } else {