In many of the articles I read supporting non-scientific ideas or in informal debates, one extremely common logical fallacy I run across is the argument from final consequences. I see it often enough that I think it deserves it own post so you can try to avoid this logical pitfall.
There are two main ways this fallacy is usually presented, but both share a common trait. They basically invert cause and effect. One way this is done is to say that if someone benefited from X, that person caused X. You can imagine in a murder investigation that if someone benefited highly from the death, they will be a suspect. The fallacy is to say that because they benefited, they must have caused it. Motivation is separate from actually committing the crime. There is a reality to the world that coincidences happen. Everyone, and in particular politicians, will try to capitalize on these coincidences, but that doesn't mean they caused them.
The other way this fallacy often comes up is to say the implications of an idea determine its truth. An example is 'Fairies must exist, because that would make the world more interesting.' Whether or not they would make the world more interesting is completely irrelevant to the question of their existence. This is the equivalent of arguing that gravity can't exist, because flying is fun. One of the most common contexts you will hear this agreement is that evolution must be wrong because otherwise life has no meaning and their would be no morality.
I should mention again that just because someone uses bad logic to defend their point, doesn't mean that their conclusion is wrong. All it means is that particular argument does not support that conclusion. I have heard people on both sides of different issues make poor logical arguments, so you need to evaluate each argument on its own merits. Bad logic should however, serve as a red flag to see if any of their other arguments hold up to strong scrutiny.