Philip Newton's journal

"On 158 occasions, electors have cast their votes for President or Vice President in a manner different from that prescribed by the legislature of the state they represented. Of those, 71 votes were changed because the original candidate died before the elector was able to cast a vote. Two votes were not cast at all when electors chose to abstain from casting their electoral vote for any candidate. The remaining 85 were changed by the elector's personal interest, or perhaps by accident. Usually, the faithless electors act alone. An exception was in 1836 when 23 Virginia electors changed their vote together." -- http://en.wikipedia.org/wiki/Faithless_elector