With out looking at it, I'm gonna take a stab and say I've gotten it a billion times before.
If you move your mouse sharply, or lift it and hit it firmly - not too hard! - on your desk/mousepad, it should fix up. I think that it is caused by a piece of foreign material getting into the laser/whatever bit of your mouse that screws it up. I've only ever encountered it in ET though, no other games or in general Windows, so it's just as likely that it's an ET issue.
It disappears for me within 4 seconds, though it's gotten me killed a billion times before.
Double check you don't have a spin bind though?