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 trendy apparel retailer Zumiez (Nasdaq: ZUMZ) are dropping like they're going out of style today, falling as much as 13.9% on very heavy volume.

So what: A cheery report of May sales with 7.8% higher same-store sales compared to the 2010 period wasn't good enough to overcome two analyst downgrades. The main motivation for those downgrades was "locking in profits," since the stock had gained some 25% since the middle of March.

Now what: Trading in and out of a growth stock like Zumiez based on short-term price changes will lead you down a dangerous and very un-Foolish road. That said, Zumiez looks expensive, with shares selling for 1.8 times trailing sales -- several times the P/S ratio seen in rivals Hot Topic (Nasdaq: HOTT) and Pacific Sunwear (Nasdaq: PSUN). Our Big Short newsletter has been shorting this stock for some time, waiting for the stock to align with more realistic expectations. A day like this shows just how sensitive this valuation is to even the slightest of bad news.

Interested in more info on Zumiez? Add it to your watchlist.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. Motley Fool newsletter services have recommended buying shares of and shorting Zumiez. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.