Presuming that a real or perceived relationship between things means that one is the cause of the other.
Otherwise referred to as confusing correlation with causation. This is one of the more obvious fallacies, but still a major contributor to magical thinking.
Most rapists read pornography when they were teenagers; therefore, pornography causes violence toward women.
Another use of this fallacy is how video games are the cause of violence. Video games must have been really popular during the crusades, Salem witch trials and Spanish inquisition.