Electoral College

[Fritz]

Basically when you cast your vote for President, you aren't voting for the candidate, rather you are voting for the slate of electors that has been set up by the party.  As GfP points out, occasionally an elector casts a ballot other than the one he was elected to cast.  I don't believe there has ever been a case where an elector has flat-out voted for the other side.  If that did happen, and it actually made a difference in the election, I'm sure that it would be challenged in Congress, the state's supreme court, the federal supreme court....it would be quite a fireworks display.  However, "faithless" electors have been more interested in making a statement than in making a difference.

[Fritz]

In an election in the 1800's, someone was elected unanimosly by all the states, however, one elector voted for the opposing canidate so that George Washington would be the only president to win a unanimous victory.

Yes, that was 1820, when James Monroe was unopposed for re-election.  It wasn't actually the "opposing candidate", there really wasn't one.  The vote was cast for John Quincy Adams, who did not actually run for President that year.

I remember in 2000, there was some talk of trying to get a few of the Florida electors to change sides.  It would have taken two to produce a tie, and three to give the election to Gore.  But of course, it didn't happen.  The only faithless elector was from the Gore side, and I'm sure that wouldn't have happened if Gore's loss wasn't inevitable.