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 youth-skewed clothing retailer Aeropostale (NYSE: ARO) dropped like they were going out of fashion today, falling as much as 25% and staying at least 20% cheaper all day long.

So what: Oh boy, where do I start? Aeropostale saw disappointing same-store sales in July, reported preliminary second-quarter results far below Street estimates, and then lowered expectations for the full year based on "a lack of balance in our merchandise assortments, as well as continued promotional and macroeconomic challenges." It's fire-sale city for both the stock and the clothes, and all of this came on the worst market day I've seen since the summer of 2008.

Now what: Fellow teen-tailers Zumiez (Nasdaq: ZUMZ) and Pacific Sunwear of California (Nasdaq: PSUN) also felt the market's wrath today, falling into brief double-digit drops. None suffered as deeply as Aeropostale, though. We're talking about the third earnings miss in the last 21 quarters here, and management is clearly ready to take arms against this sea of troubles. You would not be crazy to see a buy-in opportunity on one of Mr. Market's infamous overreactions here.

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