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."

'released/nullified synthetic event. This is a no-op function. If you must ' +
      'keep the original synthetic event around, use event.persist(). ' +
      'See https://fb.me/react-event-pooling for more information.'
    );
  });
  it('should warn if the synthetic event has been released when calling `stopPropagation`', function() {
    spyOn(console, 'error');
    var syntheticEvent = createEvent({});
    SyntheticEvent.release(syntheticEvent);
    syntheticEvent.stopPropagation();
    expect(console.error.calls.length).toBe(1);
    expect(console.error.argsForCall[0][0]).toBe(
      'Warning: This synthetic event is reused for performance reasons. If ' +
      'you\'re seeing this, you\'re accessing the method `stopPropagation` on a ' +
      'released/nullified synthetic event. This is a no-op function. If you must ' +
      'keep the original synthetic event around, use event.persist(). ' +
      'See https://fb.me/react-event-pooling for more information.'
    );
  });
  it('should properly log warnings when events simulated with rendered components', function() {
    spyOn(console, 'error');
    var event;
    var element = document.createElement('div');
    function assignEvent(e) {
      event = e;
    }
    var instance = ReactDOM.render(<div onClick={assignEvent} />, element);
    ReactTestUtils.Simulate.click(ReactDOM.findDOMNode(instance));
    expect(console.error.calls.length).toBe(0);