Is that the only video that you are having problems converting?
The url-decoded source of that video is:
When I plug that into Firefox, I eventually get an error message:
Video can't be played because the file is corrupt.
That would seem to confirm that the video on YouTube's cache server is corrupt. (From the URL, you can see that the video is being downloaded from YouTube's cache server.) The video actually playing at http://www.youtube.com/watch?v=TFGtiWKy1Tk
is probably coming from another source.
I very rarely come across videos that can't be converted. If this is happening for more than one video, then I will explore this further. Otherwise, I'll chalk it up to some kind of internal YouTube error.