Recently I have noticed that when I roll the mouse-wheel to scroll downwards in Windows XP the window occasionally scrolls upwards. This is rather annoying as it makes window scrolling visually jerky. I have seen it when using Firefox and when using Internet Explorer. I have also seen it on more than one installation of Windows XP. It would appear to be a bug.
I now think this is probably a mouse hardware problem rather than a software bug. It seems to be getting worse.
I eventually fixed this problem by using narrow strips of cardboard to scrape out a large lump of fluff that had accumulated around the axle of the mouse-wheel. So it was a hardware problem (assuming fluff counts as hardware and not software).