Nate SIlver's "The Signal and the Noise" (recommended, buy it, read it, etc.) includes a great discussion of the problem of "overfitting": When you see apparent relationships in data that aren't really there. His example:
Overfitting is a particular problem during elections, when there's always some spurious attempt to explain the election through something totally unrelated like, well, football. For instance: If the Redskins won their last home game before the election, the incumbent party will hold the White House. This has held true in 16 of the last 17 elections (2004 was the exception). It's also ridiculous example of overfitting.
But perhaps you don't want to read a long (though very good!) book on common statistical mistakes. Perhaps you prefer cartoons. In that case, the brilliant XKCD has you covered: