Did you ever try to copy localy a YouTube video that seemed interesting to you ? It happened to me few days ago. On my Mac, I solved the problem by looking in the /var/folders directory where temp copies are hold.
In order to locate more precisely I had to do the following:
touch /tmp/now
find /var/folders -newer /tmp/now -print
while the video is still loading.
Then it is just a matter of converting the flash format to the m4v format, Handbrake will do the job for you.
