Fallacies are the places where your premises or their connection to your main argument fall apart.
Here are two of the most common:
- The conclusion drawn from inadequate personal experience. Like, "Most reputable medical journals and studies have ruled out a link between MMR vaccines and Autism, but my friend's kid showed symptoms after getting a shot, so there MUST be a connection." Whether or not there is a connection is not the point; carefully designed studies use large samplings and control groups and try to rule out other variables, so you need a lot more evidence than your friend’s gut. This is a hasty generalization from a small sample, but it’s also a post hoc fallacy, or assuming causation because something happens after something else.
- A cruddy syllogism. A syllogism is the kind of proof where you take one big [true] statement, and then one smaller one, and from them draw a concluding statement. Like: All vegetables are food. Corn is a vegetable. So...Corn is a food. A cruddy syllogism is one where a false statement mucks up the flow, and renders the conclusion invalid. Like: All Italians are criminals. The Wife of Leisure is Italian. The Wife of Leisure is a criminal. Now, I may be a criminal, but you wouldn't know it by this logic, because you can hardly prove that all of us Wops are truly living lives of crime. Also, I’m only half Wop. There are lots of other ways you can screw up your syllogistic argument, but this is one of the most common, and after all, this is a leisurely approach to logic.