In U.S. history, only 157 electors have been “faithless” electors, failing to vote for the candidate their state endorsed.
Sixty-three of those came when Democratic nominee Horace Greeley died after the election in 1872 but before the electoral college convened; those 63 abstained.
The most recent incident of a faithless elector came in 2004, when one Minnesota elector voted for the same candidate for both president and vice president.