Mighty Mouse issues...

I found a cheap Black foam no reflective one from PC world, cost like £2, best mat ive ever used, and the MM loves it

 

You might want to try cleaning it. I know the whole point of optical mice is that you dont need to clean it anymore, but if you get a big bit of dust, or a hair lying the wrong way across the sensor it'll throw it off. I've got a laser mouse, which is supposedly even better, but a hair got stuck under once, and it stopped moving horizontally, and would only move vertically. Until I found the hair and removed it.
Stupid thing.

 

mine started doing that earlier tonight, I thought it was the battery so I checked that, but it was still on the green.
I turned it off and on again and it seemed to work fine..

 

I use a regular white piece of paper. It doesn't collect dust like my desk would. And it doubles as a notepad.

I double the paper so it doesn't make the hard scraping noise when i had it on the desk.

 

Maybe the hair fell off when you picked it up to turn it off

Not a bad idea, I'm always looking about for something to scribble notes on, and I really can't be bothered with a mouse pad.