Formal Fallacy: Affirming the consequent - by Mike Moum
Given a statement of the form, "if x, then y", "x" is called the premise, and "y" is called the consequent, or conclusion. The statement says that if "x" is true, then "y" is also true. This is a logically valid statement.
The fallacy of "affirming the consequent" is the logical mistake of reversing the premise and consequent: it takes the form "if y, then x" (assuming that if x, then y is true). A couple of examples will make this clear.
For instance, "if it is raining, then the sky is cloudy" is a true statement. However, reversing the statement to say "if it is cloudy, then it is raining", is not necessarily true. We've all seen cloudy days without rain.
As another example, the statement "if it is cloudy, the sun is not shining" is also a true statement. However, reversing the premise and consequent, "if the sun is not shining, it is cloudy" is not true. It could be nighttime, for example.
Affirming the consequent is a formal logical fallacy. An explanation with more examples can be found here: http://rationalwiki.org/wiki/Affirming_the_consequent