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 teen-oriented apparel retailer Tilly's (NYSE:TLYS) plummeted 19% today after its quarterly results and guidance missed Wall Street expectations.

So what: Tilly's third-quarter profit managed to meet estimates, but a miss on the top line -- revenue of $124.9 million versus the consensus of $128.7 million -- coupled with downbeat guidance for the fourth quarter reinforces concerns over slowing demand. In fact, same-store sales increased just 1.9% during the quarter, suggesting that the brand doesn't quite have the growth potential that analysts once thought.

Now what: Management now sees full-year 2012 adjusted EPS of $0.88-$0.91, down from its prior view of $0.88-$0.94 and below Wall Street's estimate of $0.92. "While we are cautious in our outlook for the fourth quarter," CEO Daniel Griesemer said, "I am confident that the fundamentals of our business ... and the disciplined execution of our management team will enable us to make steady progress on our long-term growth initiatives." With the stock hitting a new 52-week low today and trading at a forward P/E of 12, now might even be an opportune time to buy into that bullishness.

Interested in more info on Tilly's? Add it to your watchlist.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.