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."

invariant(
      ReactTestUtils.isCompositeComponent(inst),
      'findAllInRenderedTree(...): instance must be a composite component'
    );
    return findAllInRenderedTreeInternal(ReactInstanceMap.get(inst), test);
  },
  /**
   * Finds all instance of components in the rendered tree that are DOM
   * components with the class name matching `className`.
   * @return {array} an array of all the matches.
   */
  scryRenderedDOMComponentsWithClass: function(root, classNames) {
    if (!Array.isArray(classNames)) {
      classNames = classNames.split(/\s+/);
    }
    return ReactTestUtils.findAllInRenderedTree(root, function(inst) {
      if (ReactTestUtils.isDOMComponent(inst)) {
        var className = inst.className;
        if (typeof className !== 'string') {
          
          className = inst.getAttribute('class') || '';
        }
        var classList = className.split(/\s+/);
        return classNames.every(function(name) {
          return classList.indexOf(name) !== -1;
        });
      }
      return false;
    });
  },