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.
Love the app so far... here are a few ideas. Perhaps if the new ScrnShots Desktop emulated a few other capture tools, us screenshot junkies would even need the others...
Specifically, in Skitch you can click on the title bar of any window or desktop and it grabs the whole thing - really quick and easy. With Paparazzi, you enter a URL and give it size parameters and pow - PNG. Both of these features would greatly add to the ScrnShots Desktop yumminess.