I had a friend who started taking vitamin C, and he caught a cold. Therefore, it must have been the vitamin C which produced the cold.

Post hoc ergo propter hoc (Latin: "after this, therefore because of this") is a logical fallacy that states "Since event Y followed event X, event Y must have been caused by event X." It is often shortened simply to post hoc fallacy.

questionable cause fallacy/confusing correlation and causation