The flash crash
The best proof ever that it's not good to watch your portfolio too closely arrived on May 6. The Dow Jones Industrial Average plunged 700 points in minutes, and then retraced most of it a few minutes later.
Investors lost more confidence than cash, and returned to the shoebox-under-bed asset allocation plan they had embraced in 2009. Weirdly (or tellingly), the market slowly revisited its flash crash low one month later.