Occasionally it happens but almost always it's because the candidate misled the person or organization making the endorsement. The St. Petersburg Times quite vocally retracted their endorsement of Mel Martinez during the 2004 election when they found that he personally hired two gay rights activists as staffers while attacking Bill McCollum as "the darling of homosexual extremists". Said it looked like Martinez was a shady character who will saything to win even if he believes the opposite. I agreed.
That same year many freepers including me were supporting black Republican Vernon Robinson in NC. He was at the time slandering his opponent Virgina Foxx as a Hillary Clinton clone, clearly big time bull.
Looking at Florida endorsement retraction
http://www.sptimes.com/2004/08/30/Opinion/McCollum_for_GOP.shtml
It actually doesn’t mention gay staffers in the Martinez camp. It just objects to the “bigotry” of the ads, natch.