We can not fetch videos from this type of page.
Your URL was: http://save-video.mp4?http%3A%2F%2Fv21.lscache1.c.youtube.com%2Fvideoplayback%3Fip%3D0.0.0.0%26sparams%3Did%252Cexpire%252Cip%252Cipbits%252Citag%252Cburst%252Cfactor%26itag%3D18%26ipbits%3D0%26signature%3DB97A3567F691A248C99B7A1ECF4BA57F106FFE5A.4E0FB9DC1A6087C785FA3A52D40247B9BFA5F14A%26sver%3D3%26expire%3D1252630800%26key%3Dyt1%26factor%3D1.25%26burst%3D40%26id%3D6fa6177d59797e14
The URL must be like http://www.youtube.com/watch?v=KPtX7hak0Qs
https://www.
and replace with pwn
Example: