Some weird things from downloading a vid from Youtube

Hallo, i just wanna ask, if someone have similar problem with me.

Is it just me, sometimes i watch vid while downloading them in Youtube. And, after i finish the download, somehow, the youtube site that content my played video link to a blank page, but i success in downloading the vid. But it really annoying, i gotta wait for the buffer again to watch the vid. Someone, help!