1. Let a little bit of a torrent's files download.
2. Set all files to "Don't Download", except for one file.
3. uTorrent will show that the torrent is at 100%, and will stop downloading, even though the one file is not completely downloaded.
This may require that the other files that were briefly downloaded have a completed size that exceeds the size of the one file, I'm not sure about that. My guess is that it's comparing the completed size to the size of the only file that needs to be downloading, and seeing that it is larger, thinks that it's work is done. The worst part is that it puts the one file in the "completed" folder, even though it is not completed.