From my reading of the news Election Day seems pretty shenanigan-free so far, but people in Philly reported that their computerized straight-Dem tickets were canceling out their votes for Obama. As a geek, the explanation totally didn’t surprise me:

This is because it will be as if you had pushed the Obama button twice: the first time selecting him and the second time unselecting him. If you want to vote the straight dem ticket, then just push that button and it will include Obama.