To reproduce, open http://jsfiddle.net/0jjnaghw/
Error is logged to browser dev tools (in Chrome & Firefox, somehow not in Safari).

Happens on v15.0.0-rc.2 and on latest master version.

The assignment of additional event data to the newly created SyntheticEvent triggers a console error:
https://github.com/facebook/react/blob/3a4e1dbb5b2957875cab6af11a5e5934e125a0d8/src/test/ReactTestUtils.js#L502

> "Warning: This synthetic event is reused for performance reasons."

},
    _replaceNodeWithMarkup: function() {},
    _renderValidatedComponent:
      ReactCompositeComponent.Mixin
        ._renderValidatedComponentWithoutOwnerOrContext,
  }
);
ReactShallowRenderer.prototype.render = function(element, context) {
  invariant(
    ReactElement.isValidElement(element),
    'ReactShallowRenderer render(): Invalid component element.%s',
    typeof element === 'function' ?
      ' Instead of passing a component class, make sure to instantiate ' +
      'it by passing it to React.createElement.' :
      ''
  );
  invariant(
    typeof element.type !== 'string',
    'ReactShallowRenderer render(): Shallow rendering works only with custom ' +
    'components, not primitives (%s). Instead of calling `.render(el)` and ' +
    'inspecting the rendered output, look at `el.props` directly instead.',
    element.type
  );
  if (!context) {
    context = emptyObject;
  }
  ReactUpdates.batchedUpdates(_batchedRender, this, element, context);
  return this.getRenderOutput();
};
function _batchedRender(renderer, element, context) {