Affirming the Consequent

Stating that something implies something else because the latter implies the former.
Mistaking a necessary condition for a sufficient condition.

General form:
"A implies B and B is true, therefore A is true."
"A implies B, therefore B implies A."

Typical example:
"All apples are fruits; this is a fruit, therefore this is an apple."

A necessary condition is not automatically also a sufficient one. When something implies something else, this means that the former being true makes the latter also true, not the other way around.