I've now lived through 2 different economic bubbles (and bubble bursts) as well as the election of 5 different presidents (where I could vote).  These experiences have led me to conclude that the 6 most dangerous words in the English language are:
"This time it will be different"

I love this web posting  from "The Daily Kos" that explains this chart for two reasons:  It helps provide historical context to what is going on in the stock market today.  Its an old adage, but of course the only way to keep from repeating the mistakes of the past is to study history.  This chart give great historical context.  The other reason I like it is because a picture is worth a thousand words and I appreciate creative representation of data to communicate information of value.  The picture itself isn't that unique but what it conveys is.