Occasionally, Flickr Uploadr might get stuck on a process for example trying to upload a file that is too large, one that no longer exists or a corrupt file. If so it might also not intelligently skip that file and might just keep on going and crash before you can cancel the process. If you can't stop it and it crashes over and over there is still a solution (I had to look it up and so am passing this along).

In Windows, the solution is to delete a file called photos.json.

This file is located in the Documents and Settings/[your user name]/Application Data/Flickr/Uploadr/Profile/ directory. (or do a search).

Delete that file and restart the Uploadr and it will regenerate a new photos.jsn file and won't have those problem items in its queue.