As most of us can remember from the 2000 election, Florida had lots of issues with hanging chads and butterfly ballots, and the election wasn't finally resolved until nearly a month after Election Day. In 2004, it was Ohio's turn to have lots of problems, such as hours-long lines to vote in some places (although Bush would've won Ohio anyway). In 2008 we didn't hear as much about these problems, because the election wasn't very close...but in 2012 it looks like we're heading for another very close election, where we'll inevitably hear lots more about voting irregularities from whichever side loses.
So just for fun, let's play a game: Which state do you think will screw up the election this time around and ruin it for the other 49 states? I'm going to guess Colorado.