http://en.wikipedia.org/wiki/Affirming_the_consequent

Affirming the consequent, sometimes called converse error or fallacy of the converse, is a formal fallacy of inferring the converse from the original statement. The corresponding argument has the general form:

If P, then Q.
Q.
Therefore, P.

An argument of this form is invalid, i.e., the conclusion can be false even when statements 1 and 2 are true. Since P was never asserted as the only sufficient condition for Q, other factors could account for Q (while P was false).[1]

To put it differently, if P implies Q, the only inference that can be made is non-Q implies non-P. (Non-P and non-Q designate the opposite propositions to P and Q.) Symbolically:

(P ⇒ Q) ⇔ (non-Q ⇒ non-P)

The name affirming the consequent derives from the premise Q, which affirms the "then" clause of the conditional premise.