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 SimpleComponent = React.createClass({
      getInitialState: function() {
        return {clicked: false};
      },
      handleUserClick: function() {
        this.setState({ clicked: true });
      },
      render: function() {
        return (
          <div
            ref={() => {}}
            onClick={this.handleUserClick}
            className={this.state.clicked ? 'clicked' : ''}
          />
        );
      },
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    var result = shallowRenderer.getRenderOutput();
    expect(result.type).toEqual('div');
    expect(result.props.className).toEqual('');
    result.props.onClick();
    result = shallowRenderer.getRenderOutput();
    expect(result.type).toEqual('div');
    expect(result.props.className).toEqual('clicked');
  });
  it('can setState in componentWillMount when shallow rendering', function() {
    var SimpleComponent = React.createClass({
      componentWillMount() {