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('traverses children in the correct order', function() {
    var Wrapper = React.createClass({
      render: function() {
        return <div>{this.props.children}</div>;
      },
    });
    var container = document.createElement('div');
    ReactDOM.render(
      <Wrapper>
        {null}
        <div>purple</div>
      </Wrapper>,
      container
    );
    var tree = ReactDOM.render(
      <Wrapper>
        <div>orange</div>
        <div>purple</div>
      </Wrapper>,
      container
    );
    var log = [];
    ReactTestUtils.findAllInRenderedTree(tree, function(child) {
      if (ReactTestUtils.isDOMComponent(child)) {
        log.push(ReactDOM.findDOMNode(child).textContent);
      }
    });
    
    expect(log).toEqual(['orangepurple', 'orange', 'purple']);