It happens to me too occasionally, but I never questioned it, just cursed and wiggled it around (and giggled a couple of times like a baby gazing at a crib-mobile-maybe even gurgled too).
It only happens randomly and only when I'm not touching the mouse.
![Smiley](/smf/Smileys/default/smiley.gif)
teehee
(I use an un-shiney matte wood surface, I hate mousepads. I feel the boundaries and get annoyed at slipping off of it's edge
![Smiley](/smf/Smileys/default/smiley.gif)
)