Consider the following:
<a href="/test.html" onclick="event.preventDefault();return false;">Test</a>
When you click the link in this example, absolutely nothing should happen. And that is exactly the case with all versions of Safari, Chrome, FireFox, and IE <= 10. IE11 introduces a bug where when you click the link, a request gets sent to test.html (nothing visually happens in the browser, but in reality behind the scenes IE11 is sending an HTTP request to the href of the anchor).
We use the HTML5 History API for ajax and bind handlers to anchor onclick events, and this is causing all kinds of bad things to happen -- items being added to cart twice, unnecessary load on our server, and unnecessary delay on the client in IE11 browsers because resources are being requested when they shouldn't be?