On November 7, 2006, there was an electoral disaster in Sarasota County, Florida. Almost 18,000 people, about one in seven of the people who voted electronically, left the polls without recording a vote in the congressional race, the hottest race on the ballot. Most observers agree that few of these voters deliberately skipped voting in that race. Instead, they were either misled into not seeing that race or the voting machines somehow failed to record their votes. There is consensus on one point, however. Although Republican Vern Buchanan was certified the winner by only 369 out of more than 238,000 votes and is now representing the 13th Congressional District of Florida (CD13) in the U.S. House of Representatives, if the “missing” votes had been recorded, Democrat Christine Jennings would almost certainly have been elected (Stewart, 2006).
See David Jefferson's account of his findings here. Jefferson is a computer scientist at Lawrence Livermore National Laboratory and chair of the California Secretary of State Voting Systems Technology Assessment and Advisory Board.