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."

module.prototype.render.mockImplementation(function() {
      return React.createElement(
        mockTagName,
        null,
        this.props.children
      );
    });
    return this;
  },
  /**
   * Simulates a top level event being dispatched from a raw event that occurred
   * on an `Element` node.
   * @param {Object} topLevelType A type from `EventConstants.topLevelTypes`
   * @param {!Element} node The dom to simulate an event occurring on.
   * @param {?Event} fakeNativeEvent Fake native event to use in SyntheticEvent.
   */
  simulateNativeEventOnNode: function(topLevelType, node, fakeNativeEvent) {
    fakeNativeEvent.target = node;
    ReactBrowserEventEmitter.ReactEventListener.dispatchEvent(
      topLevelType,
      fakeNativeEvent
    );
  },
  /**
   * Simulates a top level event being dispatched from a raw event that occurred
   * on the `ReactDOMComponent` `comp`.
   * @param {Object} topLevelType A type from `EventConstants.topLevelTypes`.
   * @param {!ReactDOMComponent} comp
   * @param {?Event} fakeNativeEvent Fake native event to use in SyntheticEvent.
   */