In the 2016 election, ten members of the Electoral College either voted for or attempted to vote for a different candidate than who they were pledged.
These “faithless electors” are rare, with the most recent before 2016 being in the 2004 election when there was a single faithless elector. There was a single faithless elector in 2000, and you have to go all the way back to 1988 to find another case where an elector defected.
A number of states have rules on the books penalizing faithless electors. Some (like North Carolina) carry a fine, but most states penalizing faithless electors simply cancel their vote. And according to the latest SCOTUS ruling, those penalties are constitutional. MORE