I wrote a little file upload swf in AS3 for a local printing company. The purpose was to allow their clients to upload multiple large files through their site with thumbnail generation and a nice little progress bar for each file. It worked great until Flash Player 10 came out.
After that they started getting complaints from clients that it wouldn't work. All my testing in FF, Safari, and IE 6-8 went without a hitch. Turns out, the error only appears in Firefox for Mac. Hrrr.
Apparently the Mac-FF-Player doesn't return event information properly, so FileReference info ends up being NULL. This affects the ProgressBar component as well, as it uses the FileReference instance as the source.
According to this bug report on Mozilla, it looks like the issue will be fixed with the release of Flash Player 10.1, which is currently in Beta 2.
Until that is released, be prepared to have an alternate upload form for Mac/FF users when you use the FileReference class!