Edit a decade later: The solution below this one is easier!
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Down is the most common way to scroll the mouse, so it gets much dirtier than the other directions. I've seen this problem with many Mighty Mice, and the solution is simple:
'''Use rubbing alcohol to clean the scroll ball.''' Dab some on with your finger, or perhaps use a paper towel. Roll the ball around for a while to get the alcohol spread around, and then dab it off. '''Repeat a few times''', until the mouse is scrolling correctly. Don't drown the thing, but be somewhat generous.
Down is the most common way to scroll the mouse, so it gets much dirtier than the other directions. I've seen this problem with many Mighty Mice, and the solution is simple:
'''Use rubbing alcohol to clean the scroll ball.''' Dab some on with your finger, or perhaps use a paper towel. Roll the ball around for a while to get the alcohol spread around, and then dab it off. '''Repeat a few times''', until the mouse is scrolling correctly. Don't drown the thing, but be somewhat generous.