Affirming the consequent

If A, then B.
B.
Therefore, A.

Deductively speaking, this is an invalid argument. Wrong. Bad. It is also a commonly asserted deduction in human debate.

Consider:

If this were a good writeup, it would have lots of votes.
This writeup has lots of votes.
Therefore, this is a good writeup.

The problem is that there are other possibilities. If the writeup were written by God, for instance, and it said "Vote for this writeup or you will go to hell."

(Note: This type of argument becomes valid if and only if you use a biconditional if.)

Here's one that's a bit more obvious:

If God exists, then sunsets are beautiful.
Sunsets are beautiful.
Therefore, God exists.

A logical fallacy in which, as artfuldodger clearly states, the implication logical operator is abused. The base problem here is that an implication says only that if A is true, then B must be true. If A were false, B could be either true or false.

To prove that the consequent is being affirmed, show that the relation is an implication and the premise corresponds to B. You may do this by showing another possible cause.