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."

var instance = new ShallowComponentWrapper(element);
    instance.mountComponent(transaction, null, null, context);
    this._instance = instance;
  }
};
/**
 * Exports:
 *
 * - `ReactTestUtils.Simulate.click(Element/ReactDOMComponent)`
 * - `ReactTestUtils.Simulate.mouseMove(Element/ReactDOMComponent)`
 * - `ReactTestUtils.Simulate.change(Element/ReactDOMComponent)`
 * - ... (All keys from event plugin `eventTypes` objects)
 */
function makeSimulator(eventType) {
  return function(domComponentOrNode, eventData) {
    var node;
    invariant(
      !React.isValidElement(domComponentOrNode),
      'TestUtils.Simulate expects a component instance and not a ReactElement.' +
      'TestUtils.Simulate will not work if you are using shallow rendering.'
    );
    if (ReactTestUtils.isDOMComponent(domComponentOrNode)) {
      node = findDOMNode(domComponentOrNode);
    } else if (domComponentOrNode.tagName) {
      node = domComponentOrNode;
    }
    var dispatchConfig =
      EventPluginRegistry.eventNameDispatchConfigs[eventType];
    var fakeNativeEvent = new Event();
    fakeNativeEvent.target = node;