specifically the wired optical wheel mice. occasionally the page you are viewing will jump up or down by, incidentally, the amount that one turn of the mousewheel will produce, so I suspect the mouse is sending the wheelmove signal without the wheel being moved.
the reason I narrow this to MS mice is that it happens on 3 machines at work, all with these mice, but it never happens at home where I have a logitech mouse.
I’ve seen the issue on my optical mouse (with a wheel) and it’s not Microsoft. Someone at work had the same issue, and he was using a Mac. Took me awhile to diagnose, since I hadn’t realized you could get a Mac mouse with a wheel.
My impression is that the wheel turns some sort of geared device (which is why you can feel it click as it moves and why the pages don’t scroll completely smoothly). Essentially, the wheel seem to be balanced on one of the points of the gear and is vibrating back and forth.