One common logical fallacy is known as ‘affirming the consequent’. Arguments that commit this fallacy have this general form:
If P then Q.
(In technical terminology, P is the antecedent of the first, conditional premise and Q is the consequent of that premise. The second premise of the argument affirms the consequent of the first premise rather than its antecedent; hence the fallacy of ‘affirming the consequent’.)
It isn’t difficult to see that such arguments are fallacious, as this example makes plain:
If Bob lives in Chicago then Bob lives in America.
Bob lives in America.
Therefore Bob lives in Chicago.