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."

if (!ReactElement.isValidElement(inst)) {
      return false;
    }
    
    
    var prototype = inst.type.prototype;
    return (
      typeof prototype.render === 'function' &&
      typeof prototype.setState === 'function'
    );
  },
  isCompositeComponentElementWithType: function(inst, type) {
    var internalInstance = ReactInstanceMap.get(inst);
    var constructor = internalInstance
      ._currentElement
      .type;
    return !!(ReactTestUtils.isCompositeComponentElement(inst) &&
             (constructor === type));
  },
  getRenderedChildOfCompositeComponent: function(inst) {
    if (!ReactTestUtils.isCompositeComponent(inst)) {
      return null;
    }
    var internalInstance = ReactInstanceMap.get(inst);
    return internalInstance._renderedComponent.getPublicInstance();
  },
  findAllInRenderedTree: function(inst, test) {
    if (!inst) {
      return [];
    }