Look out for unjustified inferences. An inference is a general conclusion derived from known facts. Three common errors with this process are (1) accepting information as factual when it is not, (2) assuming that an example is representative of the larger population, and (3) making character inferences based on situational factors. Consider the following example:
People are stupid. Why? Because this one time, at band camp, this kid jumped off the roof attempting to use his tuba as a parachute.
First, did this really happen (is there really even a band camp??) Second, the stupidity of one kid can hardly be said to be representative of the entire human race. Third, can we really conclude that the kid in the story has a subpar intellectual disposition because of this one example? Perhaps it was a low roof, he was confident that he wouldn't get hurt, and he won $100 bucks from his friends for doing it.
Inferences are vital to reasoning and scientific study. Don't avoid making them, just make sure they are justified.