Association fallacy, or "Guilt by Association", is when two concepts or things are equated, or have equal attributes, due to a common held attribute, when the comparison is not true. This error arises through casual thinking, since by nature, we tend to categorize what we perceive from reality to a high degree. To a degree, it's similar to a "correlation does not equal causation" error. Some examples are:
- Bob's car is fast. Bob's car is also red. Therefore, my red car is also fast.
- Fred is smart. Fred is a Christian. Therefore, all smart people are Christian.
- Chocolate is made from organic compounds, and is tasty. Poop is also made from organic compounds, and is therefore, tasty.
During debates, atheists will hear a number of these examples:
- Stalin was immoral. Stalin was an atheist. You are an atheist, like Stalin, and therefore are immoral too.
- Religions are people that get together and discuss common topics. Your atheist community gets together and discusses common topics, and is therefore just another religion.
- People of faith are fervent in their beliefs. Bob as an atheist is fervent about his beliefs, and thus has faith too.
- "Remember, Satan was the first to demand equal rights." 
Often this fallacy can be countered with other associations that are even more ludicrous. For example, it is common for Christians to point out the evils done by the "atheists" Adolf Hitler, Joseph Stalin and Pol Pot. All three of them have an 'o' in their first name, so anyone with an 'o' in their first name is likely to commit atrocities. Another simple way to reverse it is to "associate" the murderous tendencies of famous dictators or movements, like the KKK, Adolf Hitler (he was raised a Catholic), the Church during the Dark Ages and Inquisition era etc. with their religious faith in Christianity, just as validly. Expect the No true scotsman or Special pleading fallacies to intervene here.