This usually means that the user mixed up our lowercase/uppercase naming convention; see https://github.com/facebook/react/pull/3429#issuecomment-85166440.

We could add a message like

> Warning: The tag <commentBox> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter.

We'd check this only once per tag and only in development mode.

it('should throw when an attack vector is used', () => {
      var hackzor = React.createElement('div><img /><div');
      expect(() => ReactTestUtils.renderIntoDocument(hackzor)).toThrow();
    });
  });
  describe('nesting validation', () => {
    it('warns on invalid nesting', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(<div><tr /><tr /></div>);
      var addendum = ReactDOMFeatureFlags.useFiber
        ? '\n    in tr (at **)' + '\n    in div (at **)'
        : ' See div > tr.';
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: validateDOMNesting(...): <tr> cannot appear as a child of ' +
          '<div>.' +
          addendum,
      );
    });
    it('warns on invalid nesting at root', () => {
      spyOn(console, 'error');
      var p = document.createElement('p');
      ReactDOM.render(<span><p /></span>, p);
      var addendum = ReactDOMFeatureFlags.useFiber
        ? 
          '\n    in p (at **)' + '\n    in span (at **)'
        : ' See p > ... > p.';
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: validateDOMNesting(...): <p> cannot appear as a descendant ' +