I'm not sure how it works elsewhere (I'm in Texas), but when I voted in November, they gave me a 4-digit number to enter when I first walked up to the machine. If I told them "this receipt doesn't match the ballot I cast", I suppose they could type my 4-digit number into the computer and say "delete this ballot", feed the receipt into a shredder, and then give me a new 4-digit number to try again. They wouldn't need to see what my receipt says to do that.

But you could very easily forge a receipt to commit fraud,* so there would have to be some kind of biometric or cryptographic security.

*Possibly in conjunction with targeting precincts that heavily favor one party, or individual voters registered with a specific party.

Yes, you probably could. Local person votes twice, 5 people commit fraud type stuff is incredibly hard to prevent and probably happens every election. As long as it requires X people to move X votes fraudulently, I would say we are going to be OK. Can you move a local election in a small town via this method? Probably, but our election system has defense in depth (recounts, election judges, polling, every-knows-everyone, voting receipts, etc. etc) so this isn't exactly easy.

It becomes a problem when 1-10 people can change the outcome of a district+ size voting block. "Hacking a voting machine" falls into this category. If 1-10 people can change a Red state to Blue via any method, that is what we should be concerned about. Again, defense in depth applies here. They would have to do many things perfectly AND get lucky for this to even be a possibility (no recounts, no voting machine inspection, no election judges intuition, etc. etc. etc.)

If my crazy uncle wants to vote twice, eh go ahead. Its difficult to do, takes a medium amount of effort and might work or might not.

You get a receipt that show your vote. After veryfying the vote, You physically and manually put the receipt in the ballot box as with manual voting.

The physical receipts are the legally valid votes and can be counted with leisure in the days after the election. The electronic copy can be used for fast aggregation and to give instant results at the end of the voting day.

2 out of 10 voters use machines that don't have a paper trail like you suggest.

http://www.nbcnews.com/news/us-news/paperless-pennsylvania-c...