Andrew Appel is one of the best known academics studying and commenting on elections and computer science and technology related to elections. He wrote a really interesting report, "New Jersey Elections Coverup", going into great detail about a specific incident during the June 2011 New Jersey primary election where the reported outcome of a particular race was demonstrated to be incorrect.
It was a small election and so they could literally get affidavits from voters to prove this. Mr. Apple was hired as an expert (apparently he does a lot of this, which for me colors his words to some extent) to help investigate what might have happened. The report tells a compelling story.
If you are one of those who wonders whether election fraud happens or can easily happen, this report will interest you. It also adds weight to the conventional wisdom among security and computer experts a physical paper ballot, marked by the voter, and scanned by a computer is really the best and perhaps only way to ensure fair elections.