If you have a default install of Ubuntu and you enabled Desktop Effects you’ll have trouble when you try to play a video. The video itself will play just fine normally, but if you try to combine that with a couple of Compiz effects, like trying to make it a bit transparent or trying to move the video while it’s playing, or rotating the cube while the video is on you’ll get a blank screen, sometimes green, sometimes grey. The way to fix this is:

Totem:

The inbuilt player. Open a Terminal and copy paste the following command:
gstreamer-properties
Then change the default output plugin to X Window System (no xv)

mplayer:

The process is essentially the same, you change the output plugin to x11. It’ll be okay so long as you’re not using xv. If you’re using the command line interface just add -vo x11 to the end. So a command to play abc.avi would be:
mplayer abc.avi -vo x11
If you’re using the GUI, you can change the settings there.