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."

},
  isDOMComponent: function(inst) {
    return !!(inst && inst.nodeType === 1 && inst.tagName);
  },
  isDOMComponentElement: function(inst) {
    return !!(inst &&
              ReactElement.isValidElement(inst) &&
              !!inst.tagName);
  },
  isCompositeComponent: function(inst) {
    if (ReactTestUtils.isDOMComponent(inst)) {
      
      
      return false;
    }
    return inst != null &&
           typeof inst.render === 'function' &&
           typeof inst.setState === 'function';
  },
  isCompositeComponentWithType: function(inst, type) {
    if (!ReactTestUtils.isCompositeComponent(inst)) {
      return false;
    }
    var internalInstance = ReactInstanceMap.get(inst);
    var constructor = internalInstance
      ._currentElement
      .type;
    return (constructor === type);
  },
  isCompositeComponentElement: function(inst) {