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.

setAndValidateContentChildDev = function(content) {
    var hasExistingContent = this._contentDebugID != null;
    var debugID = this._debugID;
    
    var contentDebugID = -debugID;
    if (content == null) {
      if (hasExistingContent) {
        ReactInstrumentation.debugTool.onUnmountComponent(this._contentDebugID);
      }
      this._contentDebugID = null;
      return;
    }
    validateDOMNesting(null, '' + content, this, this._ancestorInfo);
    this._contentDebugID = contentDebugID;
    if (hasExistingContent) {
      ReactInstrumentation.debugTool.onBeforeUpdateComponent(
        contentDebugID,
        content,
      );
      ReactInstrumentation.debugTool.onUpdateComponent(contentDebugID);
    } else {
      ReactInstrumentation.debugTool.onBeforeMountComponent(
        contentDebugID,
        content,
        debugID,
      );
      ReactInstrumentation.debugTool.onMountComponent(contentDebugID);
      ReactInstrumentation.debugTool.onSetChildren(debugID, [contentDebugID]);
    }
  };