I don't really know how to describe this one, but it's been happening on two Ubuntu computers of mine. In some games, when I make them fullscreen, any mouse input makes it go absolutely insane and just decides to move to the left. The more I move the mouse, the more it moves to the left. This makes playing first person games absolutely impossible to play. I have no idea where to even start looking, does anyone have any suggestions? So far this has happened to me in Heavy Bullets, Interstellar Marines, and Sir, You Are Being Hunted. I thought it might be a problem with the engine some of these games run on (Unity), but I have run other Unity games just fine.
Be warned, this will change the default greeter to something much less attractive. Here are some notes on reverting that: Restore Unity Greeter