Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of apparel retailer Express, Inc. (NYSE:EXPR) plummeted 23% today after its full-year outlook disappointed Wall Street.

So what: Express' third-quarter results managed to meet estimates -- EPS increased 15% on a revenue bump of 7% -- but downbeat guidance for the rest of the year is triggering serious concerns over slowing growth going forward. In fact, management cited a sharp increase in margin-pressuring promotional activity for the downbeat view, suggesting that its competitive position is weakening. 

Now what: Management now sees full-year EPS of $1.46 to $1.51, down from its prior view of $1.52 to $1.60 and below the consensus of $1.61. "We had been planning for a promotional holiday season but we now expect the intensity of those promotions to reach heightened levels and we are updating our full year guidance accordingly," said Chairman and CEO Michael Weiss. "Our overarching goal is to continue presenting Express as a premier fashion authority for our demographic, and we believe that our current and go-forward assortments demonstrate that standing." Of course, given the strong competitive headwinds facing Express at this point, I'd wait for an even wider margin of safety before buying into that optimism.