I just saw this vid. It was posted to YouTube back in January of 2008. Don't discount the shock value of what you're going to watch just because this sworn testimony is four years old.
This man's explanation of what can be done to elections software is proof enough of why a "paper trail" is absolutely necessary when electronic balloting is in use. The voter should receive a receipt and a carbon copy kept for the people running the polling places to count against the machine totals.
Watch this and listen to what this man has to say.
Here you are:
The congressman named in this is Congressman Tom Feeney of Florida. It seems that he even had his wife in on the scam!
I can't stress enough how important it is that there be a paper trail to check against the electronic totals. Without one, your vote may be as worthless as the paper you don't cast it on!