We know that some machines were changing votes to Obama because voters caught it. How many did not and would the machine have to show you what it was doing? It would only have to happen in some key locations...like Philly where they threw out GOP poll watchers. 70+ of them. What do you think was going on there?
We also know of a case in Ohio where an election judge was tossed because he/she was letting unregistered voters vote on regular, not provisional ballots. How many of those votes were counted. We will probably never know.