http://blog.nola.com/SELU/2008/11/the_electoral_college_explaine.html
Faithless electors usually act alone but sometimes form a bloc. Fortunately, faithless electors have never changed the outcome of an election. They can be punished by the state they represent after they have cast a vote outside their pledge, but their votes will still count in determining who will win the election, except in Michigan and Minnesota, where state laws invalidate votes cast other than as pledged.
While 24 states have laws to punish faithless electors, none have actually been punished.
In 2000 1 elector from DC did not vote for Gore as required by DC statue, but the vote still stood. I don’t know what happened to her though.