There are reports of voting machines recording Democratic and other votes as votes for Republicans:
A friend of our family ours went last Friday to early vote in Maplewood at Sunnen Park. He voted for Claire McCaskill, but each time he, the election worker, and the election supervisor pressed the screen for Claire, the screen said he had voted for Jim Talent.
I sent this info along to a friend that works in Claire's campaign. I have the name and number of the guy this happened to if you are interested.
Bradblog suggests bringing video equipment when you go to vote to record incidents of this kind of thing. I'd look into whether or not that is permitted where you vote first but a witness or making a complaint about it at the polls is certainly a good idea. Make sure that it's a big enough complaint that other voters will hear and check.