The rollers both had big gummy rings on them, right where the ball touched them. OBVIOUSLY the gummy ring was there to make the ball adhere better to the roller so that the mouse would track better. Only after I looked at another mouse and discovered much smaller gummy rings did I realize that it was skin oil combined with dust mites and ringworm that builds up as the mousepad sheds!
Now those rings gross me out. I am compelled to scrape them off whenever I feel the slightest irregularity in the rolling of the ball.
I don't know why it doesn't gross me out to TOUCH the goo-rings. It should. That stuff is NASTY!