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.

if (__DEV__) {
        setAndValidateContentChildDev.call(this, null);
      }
      this.updateChildren(nextChildren, transaction, context);
    }
  },
  getHostNode: function() {
    return getNode(this);
  },
  /**
   * Destroys all event registrations for this instance. Does not remove from
   * the DOM. That must be done by the parent.
   *
   * @internal
   */
  unmountComponent: function(safely, skipLifecycle) {
    switch (this._tag) {
      case 'audio':
      case 'form':
      case 'iframe':
      case 'img':
      case 'image':
      case 'link':
      case 'object':
      case 'source':
      case 'video':
        var listeners = this._wrapperState.listeners;
        if (listeners) {
          for (var i = 0; i < listeners.length; i++) {
            listeners[i].remove();