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."

var ReactTestUtils;
describe('ReactTestUtils', function() {
  beforeEach(function() {
    React = require('React');
    ReactDOM = require('ReactDOM');
    ReactDOMServer = require('ReactDOMServer');
    ReactTestUtils = require('ReactTestUtils');
  });
  it('should have shallow rendering', function() {
    var SomeComponent = React.createClass({
      render: function() {
        return (
          <div>
            <span className="child1" />
            <span className="child2" />
          </div>
        );
      },
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    var result = shallowRenderer.render(<SomeComponent />);
    expect(result.type).toBe('div');
    expect(result.props.children).toEqual([
      <span className="child1" />,
      <span className="child2" />,
    ]);
  });
  it('should throw for invalid elements', function() {
    var SomeComponent = React.createClass({
      render: function() {