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."

/**
   * Like scryRenderedDOMComponentsWithClass but expects there to be one result,
   * and returns that one result, or throws exception if there is any other
   * number of matches besides one.
   * @return {!ReactDOMComponent} The one match.
   */
  findRenderedDOMComponentWithClass: function(root, className) {
    var all =
      ReactTestUtils.scryRenderedDOMComponentsWithClass(root, className);
    if (all.length !== 1) {
      throw new Error(
        'Did not find exactly one match (found: ' + all.length + ') ' +
        'for class:' + className
      );
    }
    return all[0];
  },
  /**
   * Finds all instance of components in the rendered tree that are DOM
   * components with the tag name matching `tagName`.
   * @return {array} an array of all the matches.
   */
  scryRenderedDOMComponentsWithTag: function(root, tagName) {
    return ReactTestUtils.findAllInRenderedTree(root, function(inst) {
      return ReactTestUtils.isDOMComponent(inst) &&
            inst.tagName.toUpperCase() === tagName.toUpperCase();
    });
  },
  /**
   * Like scryRenderedDOMComponentsWithTag but expects there to be one result,