Take a look at the ballot being used in certain counties in Ohio and tell me at a glance which holes you would punch to vote for Bush or for Kerry.
If you saw this ballot and punched out the green hole, then congratulations, you've just thrown away your vote! To actually vote for Bush, you are supposed to punch out the hole highlighted in red.
This happens to be the most egregious example I've seen, but there's more where that came from. And not always favoring Kerry, either. This is why there is bound to be a lot of litigation come Tuesday with lawyers on both sides ready to challenge votes cast on ballots like this.