Referred to as a “calibration error,” voting machines in Maryland were reported to be switching votes from Republican to Democrat. Thus far, nine counties have investigated this problem and taken effected machines out of service.
This is far from the first time such “calibration errors” have been reported. In one case in Illinois the Republican candidate's own vote for himself was switched by the machine to a vote for his Democrat opponent.
These errors have also been reported in prior election cycles, with the majority being switches from Republican to Democrat, although 2010 saw some Texas votes switched to the Green Party.
In 2012, voters complained about votes for Mitt Romney being switched to votes for Barack Obama in Ohio, Nevada, Kansas, North Carolina, Missouri and Colorado. Some have suggested that voting should be more transparent - with public access to video monitoring of election counting and machine or electronic counting of votes being done away with altogether.
Additional safeguards against election fraud could include a voter ID system (like has been implemented in India, among other countries). A forthcoming report summarized at the Washington Post estimated that as much as 14% of non-citizens are illegally registered to vote in the United States, and that an estimated 6.4% and 2.2% voted in the 2008 and 2011 elections.
While it seems small, when taken out of a universe of non-citizens in the United States running above 11 million, such percentages translate to several hundred thousands voters - more than enough to swing elections in certain districts and therefore, even nationally.