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.

};
          mountComponent({is: 'custom-shady-div'});
          expectDev(console.error.calls.count()).toBe(1);
          expectDev(console.error.calls.argsFor(0)[0]).toContain(
            'A component is using shady DOM. Using shady DOM with React can ' +
              'cause things to break subtly.',
          );
          mountComponent({is: 'custom-shady-div2'});
          expectDev(console.error.calls.count()).toBe(1);
        } finally {
          document.createElement = defaultCreateElement;
        }
      }
    });
    it('should treat menuitem as a void element but still create the closing tag', () => {
      var container = document.createElement('div');
      var returnedValue = ReactDOMServer.renderToString(
        <menu><menuitem /></menu>,
      );
      expect(returnedValue).toContain('</menuitem>');
      expect(function() {
        ReactDOM.render(<menu><menuitem>children</menuitem></menu>, container);
      }).toThrowError(
        'menuitem is a void element tag and must neither have `children` nor use ' +
          '`dangerouslySetInnerHTML`.',
      );
    });
    it('should validate against multiple children props', () => {
      expect(function() {
        mountComponent({children: '', dangerouslySetInnerHTML: ''});