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."

/**
 * @class ReactTestUtils
 */
function findAllInRenderedTreeInternal(inst, test) {
  if (!inst || !inst.getPublicInstance) {
    return [];
  }
  var publicInst = inst.getPublicInstance();
  var ret = test(publicInst) ? [publicInst] : [];
  var currentElement = inst._currentElement;
  if (ReactTestUtils.isDOMComponent(publicInst)) {
    var renderedChildren = inst._renderedChildren;
    var key;
    for (key in renderedChildren) {
      if (!renderedChildren.hasOwnProperty(key)) {
        continue;
      }
      ret = ret.concat(
        findAllInRenderedTreeInternal(
          renderedChildren[key],
          test
        )
      );
    }
  } else if (
    ReactElement.isValidElement(currentElement) &&
    typeof currentElement.type === 'function'