Optical meeces can get pretty stroppy if the surface underneath is fussy - on my desk at work I have one of those calendar-scribble pad thingies, which gets mucky and scribbled upon quite quickly. The mouse cursor jumps all over the place when I'm trying to do something intricate, and I have to strip the top sheet off to expose a clean one...!

Even when I'm not using it, strange things have been known to happen!

I have very little of such trouble here at home, except when the optical mouse gets fluff up its bung 'ole... rather than being erratic, it simply ignores me!