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.
As reported by Fox News:
The Supreme Court on Monday upheld state laws requiring those chosen for the Electoral College to back the popular winner in their state’s presidential race, a rebuke of a group of so-called “faithless” presidential electors in Washington and Colorado who sued after they were sanctioned for voting contrary to pledges they took before becoming electors.
In a 9-0 ruling, the court said that those sanctions — in Washington a fine and in Colorado being removed and replaced as an elector — are constitutional.
The cases come after a group of Democratic electors that called themselves the “Hamilton Electors” voted for moderate Republicans instead of Hillary Clinton in 2016, in an unsuccessful effort to convince Republican electors to vote for somebody besides President Trump.
While faithless electors are rare, they do theoretically have the ability to change the results of an election in a tight race (such as in 2000 when George W. Bush won by five electoral college votes.