When I debate atheists, from time to time I point out when they commit some of the standard logical fallacies. The genetic fallacy probably comes up as often as any other. This occurs when you incorrectly assume that by proving the origin of a belief, you have thereby disproven the validity of that belief. For example, atheists often claim that morality is merely the byproduct of evolutionary processes. Even if true, however, at best this may show how people arrived at their individual sense of morality. It does nothing one way or the other to demonstrate whether objective morality actually exists independent of our perceptions.
In fairness, though, if the theist is entitled to point out logical fallacies committed by the atheist, they should be permitted to do the same in return. One favorite tactic of many is to claim that in advancing the kalam cosmological argument, Christian apologists like myself commit the “fallacy of composition.”