A commonly used logical fallacy is the straw man argument. It happens when people either make up an argument, or change the subject of the argument. This argument gets its name from the practise of using straw men as opponents in jousting practise. It symbolises the act of putting a fake enemy in place, which is what this kind of argument does.
A common straw man argument is this one against evolution:
If people evolved from chimpanzees, then why aren’t chimpanzees still evolving into people.”
This is obviously not attacking a statement made by evolution. Evolutionists claim that people are related to chimpanzees via a common ancestor, so this argument is invalid. Their reasoning comes down to arguments based on similarity, which are illogical.
A good example of changing the subject can be found here. In the argument behind that blue text, the commentor doesn’t argue the issue at all, which is subjective morality, which he supports, he argues against Christianity, not for a subjective morality, or against an objective morality.
Straw men arguments are common, I was once told that as a young earth creationist, that I believe that all the layers in the earth were laid down at creation, with the fossils in them. Obviously this isn’t what YECs believe, they believe the fossils and strata were laid down in the flood.
Another example would be the labels in abortion debates. In these debates, you have pro-lifers, and pro-choicers, both of these, apart from being ad hominum, actually imply straw man arguments. People who accept abortion are not anti life, any more than people who oppose abortion are anti choice. They just have different views on when a life is formed, or who gets to choose.
Another straw man is any argument against Big Bang involving a giant explosion or something from nothing. Big bang was said to be a rapid expansion from a singularity, not an explosion from nothing.
The problem with straw man arguments is that they show a lack of ability on the side of the person using them to argue the point under dispute. This causes them to start changing the subject, or putting words in their opponents mouth in order to avoid showing there lack of ability, when most of the time, the things they are arguing against, could actually be debated on relevant points.