One of the most common logical fallacies is called "Affirming the Consequent." Such fallacies take the form:
If A, then B.
The first two parts (if A, then B, and B) can be true. The fallacy comes in concluding that A is true.
Joe Klein offers up a textbook example today:
Newt Gingrich is clearly running for President. How do I know? He gets dumb and angry when running for office. When not running for office, he'll take the occasional independent stand--in opposition to the teaching of creationism in science class (it's ok to teach it as a "philosophy" in a non-science setting, he has said) and he's had some very creative ideas about urban poverty. But he really can be a complete jerk when electorate politics is dominating his frontal cortex.
Let's break down Klein's argument:
If Gingrich is running for office, then he will act dumb and angry.
Gingrich is acting dumb and angry.
Therefore, Gingrich is running for office.
Klein proceeds to elaborate that Gingrich will occasionally say something rational when not running for office. Even if we assume that this is true, this is a far weaker claim than saying that Gingrich will exclusively say rational things when not running for office. And that strong claim is necessary to infer that Gingrich is running for office from the fact that he said something dumb and angry. My view is that Gingrich says dumb, angry things constantly and without regard to electoral ambitions. He is a dumb, angry man.