The scroll wheel on my IntelliMouse is clogged up with junk. I cleaned it out somewhat, but it still catches every page or so when scrolling downward (but not upward). Reminds me of the fun we had before optical mice, except I can't remove the scroll wheel to clean it.
Some kind of laptop-like trackpad with no moving parts in the middle of the mouse would be kinda cool. I hate trackpads as pointing devices, but as scrolling devices they're actually kinda neat.