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 renderedComponent = ReactTestUtils.renderIntoDocument(<Wrapper />);
    var scryResults1 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      'x y'
    );
    expect(scryResults1.length).toBe(1);
    var scryResults2 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      'x z'
    );
    expect(scryResults2.length).toBe(1);
    var scryResults3 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      ['x', 'y']
    );
    expect(scryResults3.length).toBe(1);
    expect(scryResults1[0]).toBe(scryResults2[0]);
    expect(scryResults1[0]).toBe(scryResults3[0]);
    var scryResults4 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      ['x', 'a']
    );
    expect(scryResults4.length).toBe(0);
    var scryResults5 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      ['x a']
    );
    expect(scryResults5.length).toBe(0);