Affirming the Consequent

An argument of the form

A → B.
B.
∴ A.

A implies B. B. Therefore, A.

In a three-part hypothetical syllogism, the proper form is
A → B.
A.
∴ B.

In other words, since A means B, and we have A, there must be B.

Fallacially, this would be that since A means B, seeing B we must have A. The error is in assuming that just because we get B when when we have A, that the only way to get B is with A.

This is the converse of the fallacy of Denying the Antecedent.