While counter factual history is certainly a fools errand, and you may get some less than researched and factual evidence in these responses, it is always fascinating to look back and contemplate "what if?" With that said, there is one possible scenario that always stood out in my mind as scarily plausible, but thankfully never happened.

Had the civil rights movement for African Americans in the United States taken a drastically more violent route, it is certainly possible that the ethnic conflict in the American south could have bordered on genocidal.

It is already well known that some southern whites were violent in suppressing the black population, with extra judicial executions of blacks for violations of social mores, and various murders of blacks that were excused in the legal court system. See lynchings in the United States: http://en.m.wikipedia.org/wiki/Lynching_in_the_United_States and see Emmett Till: http://en.m.wikipedia.org/wiki/Emmett_Till. Furthermore, there were and have been many race based riots in northern cities (see Detroit race riots of 1967 http://en.m.wikipedia.org/wiki/1967_Detroit_riot) and even as recently as the 1992 LA Riots (http://en.m.wikipedia.org/wiki/1992_Los_Angeles_riots).

Martin Luther King Jr. and others played an extremely pivotal role in pushing the movement towards non-violent resistance, even in the face of violent suppression from the authorities, and the competing voices of violent groups in the civil rights movement, such as the Black Panther party. Those never gained much support, and ultimately, the peaceful methods were able to accomplish many of the goals of the movement.

However, given a different "roll of the historical dice," the black population of the United States may have taken a more revolutionary stance, turning more quickly to unified violence in an attempt to achieve their goals. Terroristic bombings, the organization of militias, the seizing of military equipment, etc. We saw all of these things (perpetrated for many different reasons, not just racial) during the 20th century in the US, so a clever revolutionary mind could have found a way to organize such things en masse to attempt to force a substantial change of policy. The white population would likely respond with large amounts of lethal force against the now directly threatening population. While a genocide proper may not have been the outcome, massive outbursts of violence against the black population, akin to pogroms against Jews in Europe, may have been possible. The situation for those in revolt would have been almost certain defeat (a fact which was often used in arguments for peaceful resistance) but again, given a different roll of the historical dice, it may have been possible.

I wouldn't say it was truly a realistic possibility, but I guess there's always the slim chance that the post-Pearl Harbor Japanese-American internment camps could have taken a dark turn.

This post has been removed because it asks users to come up with hypotheticals, which we don't allow. It also asks for examples of something, which is similarly not allowed because the result is a collection of trivia-like lists of examples, not in-depth discussion.