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."

return (
            <a
              href="#"
              onClick={this.onClick}
              className={className}>
              Test link
            </a>
          );
        } else {
          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" />,
    ]);
    var updatedResult = shallowRenderer.render(<SomeComponent aNew="prop" />);
    expect(updatedResult.type).toBe('a');
    var mockEvent = {};
    updatedResult.props.onClick(mockEvent);
    var updatedResultCausedByClick = shallowRenderer.getRenderOutput();