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.

for (var event in mediaEvents) {
        if (mediaEvents.hasOwnProperty(event)) {
          inst._wrapperState.listeners.push(
            ReactBrowserEventEmitter.trapBubbledEvent(
              event,
              mediaEvents[event],
              node,
            ),
          );
        }
      }
      break;
    case 'source':
      inst._wrapperState.listeners = [
        ReactBrowserEventEmitter.trapBubbledEvent('topError', 'error', node),
      ];
      break;
    case 'img':
    case 'image':
      inst._wrapperState.listeners = [
        ReactBrowserEventEmitter.trapBubbledEvent('topError', 'error', node),
        ReactBrowserEventEmitter.trapBubbledEvent('topLoad', 'load', node),
      ];
      break;
    case 'form':
      inst._wrapperState.listeners = [
        ReactBrowserEventEmitter.trapBubbledEvent('topReset', 'reset', node),
        ReactBrowserEventEmitter.trapBubbledEvent('topSubmit', 'submit', node),
      ];
      break;