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."

event.nativeEvent; 
    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 property `nativeEvent` on a ' +
      'released/nullified synthetic event. This is set to null. 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 Proxy is supported and the synthetic event is added a property', function() {
    spyOn(console, 'error');
    var syntheticEvent = createEvent({});
    syntheticEvent.foo = 'bar';
    SyntheticEvent.release(syntheticEvent);
    expect(syntheticEvent.foo).toBe('bar');
    if (typeof Proxy === 'function') {
      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 adding a new property in the synthetic ' +
        'event object. The property is never released. ' +
        'See https://fb.me/react-event-pooling for more information.'
      );
    } else {
      expect(console.error.calls.length).toBe(0);
    }
  });
});