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."

hello, world
            </body>
          </html>
        );
      },
    });
    var markup = ReactDOMServer.renderToString(<Root />);
    var testDocument = getTestDocument(markup);
    var component = ReactDOM.render(<Root />, testDocument);
    expect(component.refs.html.tagName).toBe('HTML');
    expect(component.refs.head.tagName).toBe('HEAD');
    expect(component.refs.body.tagName).toBe('BODY');
    expect(ReactTestUtils.isDOMComponent(component.refs.html)).toBe(true);
    expect(ReactTestUtils.isDOMComponent(component.refs.head)).toBe(true);
    expect(ReactTestUtils.isDOMComponent(component.refs.body)).toBe(true);
  });
  it('should change the value of an input field', function() {
    var obj = {
      handler: function(e) {
        e.persist();
      },
    };
    spyOn(obj, 'handler').andCallThrough();
    var container = document.createElement('div');
    var instance = ReactDOM.render(<input type="text" onChange={obj.handler} />, container);
    var node = ReactDOM.findDOMNode(instance);
    node.value = 'giraffe';
    ReactTestUtils.Simulate.change(node);
    expect(obj.handler).toHaveBeenCalledWith(jasmine.objectContaining({target: node}));
  });