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 validate against multiple children props', () => {
      ReactDOM.render(<div />, container);
      expect(function() {
        ReactDOM.render(
          <div children="" dangerouslySetInnerHTML={{__html: ''}} />,
          container,
        );
      }).toThrowError(
        'Can only set one of `children` or `props.dangerouslySetInnerHTML`.',
      );
    });
    it('should warn about contentEditable and children', () => {
      spyOn(console, 'error');
      ReactDOM.render(<div contentEditable={true}><div /></div>, container);
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('contentEditable');
    });
    it('should validate against invalid styles', () => {
      ReactDOM.render(<div />, container);
      expect(function() {
        ReactDOM.render(<div style={1} />, container);
      }).toThrowError(
        'The `style` prop expects a mapping from style properties to values, ' +
          "not a string. For example, style={{marginRight: spacing + 'em'}} " +
          'when using JSX.',
      );
    });
    it('should report component containing invalid styles', () => {
      class Animal extends React.Component {