Affirming the Consequent

Fallacy
Let $p \implies q$ be a conditional statement.

Let its consequent $q$ be true.

Then it is a fallacy to assert that the antecedent $p$ is also necessarily true.

That is:
 * $p \implies q, q \not \vdash p$

Also see

 * Denying the Antecedent
 * Conditional and Converse are not Equivalent