Recently updated this plug-in and began experiencing some issues.
Firstly, it appears that there's a conflict between WP Smushit and Regenerate Thumbnails. When attempting to do a bulk regeneration, it does nothing.
Secondly, when you go to upload images via. the media uploader, multiple images fail.
I uninstalled Smush.it and this relieved the problems I experienced.
Could this be anything to do with the experimental bulk smush?