Someone who is "not a racist" suddenly going on a racist tirade in the presence of an individual or individuals who are of the ethnic background that the racist tirade is directed toward. Such an occurence was first discovered when Michael Richards (aka Kramer from Seinfeld) went on a racist tirade during a stand-up in which he verbally attacked a group of porch-monkeys that had been heckling him. Hence the name "Kramer Moment."
After Oswald soundly defeated his white friend, Irving, in a game of backgammon, Irving had a Kramer Moment in which he reffered to his black friend, Oswald, as "stupid nigger". Oswald immediately responded by pulling out a gat and busting a cap in Irving's scrawny white ass.