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."

/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ReactTestUtils
 */
'use strict';
var EventConstants = require('EventConstants');
var EventPluginHub = require('EventPluginHub');
var EventPluginRegistry = require('EventPluginRegistry');
var EventPropagators = require('EventPropagators');
var React = require('React');
var ReactDOM = require('ReactDOM');
var ReactDOMComponentTree = require('ReactDOMComponentTree');
var ReactElement = require('ReactElement');
var ReactBrowserEventEmitter = require('ReactBrowserEventEmitter');
var ReactCompositeComponent = require('ReactCompositeComponent');
var ReactInstanceMap = require('ReactInstanceMap');
var ReactUpdates = require('ReactUpdates');
var SyntheticEvent = require('SyntheticEvent');
var emptyObject = require('emptyObject');
var findDOMNode = require('findDOMNode');
var invariant = require('invariant');
var topLevelTypes = EventConstants.topLevelTypes;
function Event(suffix) {}