I have the problem on Windows 7, latest Flash version, both with IE8 and Firefox 3.6.15 To reproduce, go to this page: http://www.uploadify.com/documentation/options/auto/ Select a large file (like 1 or 2MB) for upload, wait a few seconds to make sure the upload is actually started, then click the X to cancel. On my machine, the browser freezes for about 30 seconds, then either Flash crashes or the upload completes without cancelling.