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.

var container = document.createElement('div');
      ReactDOM.render(<div foo="bar" />, container);
      expectDev(console.error.calls.count(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown prop `foo` on <div> tag. Remove this prop from the element. ' +
          'For details, see https://fb.me/react-unknown-prop\n    in div (at **)',
      );
    });
    it('should group multiple unknown prop warnings together', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div foo="bar" baz="qux" />, container);
      expectDev(console.error.calls.count(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown props `foo`, `baz` on <div> tag. Remove these props from the element. ' +
          'For details, see https://fb.me/react-unknown-prop\n    in div (at **)',
      );
    });
    it('should warn for onDblClick prop', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div onDblClick={() => {}} />, container);
      expectDev(console.error.calls.count(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown event handler property onDblClick. Did you mean `onDoubleClick`?\n    in div (at **)',
      );
    });
    it('should not warn for "0" as a unitless style value', () => {
      spyOn(console, 'error');
      class Component extends React.Component {