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."

it('should change the value of an input field in a component', function() {
    var SomeComponent = React.createClass({
      render: function() {
        return (
          <div>
            <input type="text" ref="input" onChange={this.props.handleChange} />
          </div>
        );
      },
    });
    var obj = {
      handler: function(e) {
        e.persist();
      },
    };
    spyOn(obj, 'handler').andCallThrough();
    var container = document.createElement('div');
    var instance = ReactDOM.render(<SomeComponent handleChange={obj.handler} />, container);
    var node = ReactDOM.findDOMNode(instance.refs.input);
    node.value = 'zebra';
    ReactTestUtils.Simulate.change(node);
    expect(obj.handler).toHaveBeenCalledWith(jasmine.objectContaining({target: node}));
  });
  it('should throw when attempting to use ReactTestUtils.Simulate with shallow rendering', function() {
    var SomeComponent = React.createClass({
      render: function() {
        return (
          <div onClick={this.props.handleClick}>
            hello, world.
          </div>