Drupal 7.14 image upload unstable
I believe you are experiencing a problem that Plupload has with PHP 5.3.x. I built a site for a client that uses photo galleries (Media and Media Gallery), and via the Pupload Integration Module galleries could be built by simple drag-and-drop of multiple image files. The site is hosted on Rackspace Cloud, which recently upgraded from PHP 5.2.x to PHP 5.3.14. That's when the problems began.

There is a bug report for the Plupload Integration module that shows the error I'm now getting. If you're getting the same error message, I'd recommend that you also take part in the thread, as I just did.