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 travel website operator Expedia (NASDAQ:EXPE) plummeted a whopping 27% today after its quarterly results and outlook disappointed Wall Street.

So what: The stock has been flat in 2013 on nervousness about increasing competition in the space, and today's second-quarter results -- EPS fell 28% to $0.64 on revenue growth of $1.21 billion -- only reinforce those concerns. In fact, selling and marketing expenses during the quarter jumped a whopping 33%, to $590.5 million, suggesting that it's costing Expedia more and more to battle red-hot rivals like and TripAdvisor.

Now what: Management now expects full-year adjusted EBITDA to increase in the mid-to-high single digits, versus its prior view of low-double digit growth. "[W]e remain confident about our long-term strategy," CEO Dara Khosrowshahi said in a statement. "We see continued return on our core brands' technology investments, broader adoption of Expedia Traveler Preference Program, momentum in our recent acquisitions and emerging markets as well as exciting traction in one of travel's fastest growth channels -- mobile." Of course, given the strong competitive headwinds facing Expedia at this point, I'd wait for an even wider margin of safety before buying that bull talk.      

Fool contributor Brian Pacampara has no position in any stocks mentioned. The Motley Fool recommends and TripAdvisor. The Motley Fool owns shares of and TripAdvisor. 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 has a disclosure policy.