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.

});
    
    ReactDOM.render(<span />, container, function() {
      expect(this.nodeName).toBe('SPAN');
      calls++;
    });
    
    ReactDOM.unstable_batchedUpdates(function() {
      ReactDOM.render(<span />, container, function() {
        expect(this.nodeName).toBe('SPAN');
        calls++;
      });
    });
    
    ReactDOM.unstable_batchedUpdates(function() {
      ReactDOM.render(<article />, container, function() {
        expect(this.nodeName).toBe('ARTICLE');
        calls++;
      });
    });
    expect(calls).toBe(5);
  });
  it('tracks root instances', function() {
    
    expect(Object.keys(ReactMount._instancesByReactRootID).length).toBe(0);
    ReactTestUtils.renderIntoDocument(<span />);
    expect(Object.keys(ReactMount._instancesByReactRootID).length).toBe(1);
    var container = document.createElement('div');
    ReactDOM.render(<span />, container);
    expect(Object.keys(ReactMount._instancesByReactRootID).length).toBe(2);