*feature*

>Warning: A component is `contentEditable` and contains `children` managed by React. It is now your responsibility to guarantee that none of those nodes are unexpectedly modified or duplicated. This is probably not intentional.

It would help heaps if I would know _which_ component is causing this.
src="http://example.org/video"
            type="video/mp4"
            onError={e => console.error('onError called')}
          />
        </video>,
        container,
      );
      var errorEvent = document.createEvent('Event');
      errorEvent.initEvent('error', false, false);
      container.getElementsByTagName('source')[0].dispatchEvent(errorEvent);
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('onError called');
    });
    it('should not duplicate uppercased selfclosing tags', () => {
      spyOn(console, 'error');
      class Container extends React.Component {
        render() {
          return React.createElement('BR', null);
        }
      }
      var returnedValue = ReactDOMServer.renderToString(<Container />);
      expect(returnedValue).not.toContain('</BR>');
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<BR /> is using uppercase HTML.',
      );
    });
    it('should warn on upper case HTML tags, not SVG nor custom tags', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(