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."

expect(updatedResultCausedByClick.type).toBe('a');
    expect(updatedResultCausedByClick.props.className).toBe('was-clicked');
  });
  it('can access the mounted component instance', function() {
    var SimpleComponent = React.createClass({
      someMethod: function() {
        return this.props.n;
      },
      render: function() {
        return <div>{this.props.n}</div>;
      },
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    shallowRenderer.render(<SimpleComponent n={5} />);
    expect(shallowRenderer.getMountedInstance().someMethod()).toEqual(5);
  });
  it('can shallowly render components with contextTypes', function() {
    var SimpleComponent = React.createClass({
      contextTypes: {
        name: React.PropTypes.string,
      },
      render: function() {
        return <div />;
      },
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    var result = shallowRenderer.render(<SimpleComponent />);
    expect(result).toEqual(<div />);
  });
  it('can shallowly render components with ref as function', function() {