I have lots of logos in my media library. When I view "thumbnails" on the media library page, some logos don't have a medium and large thumbnail because their dimensions are too small.
First, I would suggest that there should be a file check to see if they exist, and hide these sizes if not.
On the other hand, if I click on missing thumbnail, and then move to step 2, an error is generated, but when I refresh, the plugin has generated a blown-up version of that size. This is kind of cool -- so long as the error messages could be suppressed.