This has only happened a few times and it seems to correspond with slowness or non-responsiveness on scrnshots.com.
Once in awhile the desktop client will "freeze up" when a screenshot is uploaded. It won't activate either button in the promt (Close or View Screenshot) - they both stay inactive - there is now way out. The app won't quit either, must reboot OS X to make it reset. Weird.
When I take a screenshot from the windows client it adds an opaque layer to my entire computer and then I basically carve out what I want to take the screenshot of (crop), however my screenshot comes out still with a lighter opaque layer.