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 {!ReactComponent} The one match.
   */
  findRenderedComponentWithType: function(root, componentType) {
    var all = ReactTestUtils.scryRenderedComponentsWithType(
      root,
      componentType
    );
    if (all.length !== 1) {
      throw new Error(
        'Did not find exactly one match (found: ' + all.length + ') ' +
        'for componentType:' + componentType
      );
    }
    return all[0];
  },
  /**
   * Pass a mocked component module to this method to augment it with
   * useful methods that allow it to be used as a dummy React component.
   * Instead of rendering as usual, the component will become a simple
   * <div> containing any provided children.
   *
   * @param {object} module the mock function object exported from a
   *                        module that defines the component to be mocked
   * @param {?string} mockTagName optional dummy root tag name to return
   *                              from render method (overrides
   *                              module.mockTagName if provided)
   * @return {object} the ReactTestUtils object (for chaining)
   */
  mockComponent: function(module, mockTagName) {
    mockTagName = mockTagName || module.mockTagName || 'div';