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 consumer-review website operator Angie's List Inc (NASDAQ:ANGI) plummeted 19% Thursday after its quarterly results and outlook missed Wall Street expectations.

So what: Angie's List shares have been walloped over the past year on concerns over rapidly rising costs, and Thursday's Q2 results -- net loss widened to $18.4 million from $14.3 million in the year-ago period -- coupled with downbeat guidance only reinforce those worries. While revenue and total paid memberships both jumped more than 30% over the year-ago period, company selling expenses spiked 38% while marketing costs increased 28%, prompting several analysts to downgrade the stock on increased skepticism over the business model. 

Now what: Management now sees Q3 revenue of $80.5 million-$82.5 million -- well below the consensus of $86.6 million -- with marketing expenses in the range of $20 million-$23 million. "While revenue growth in e-commerce was lower than expected, we continued to make progress against our strategy of enabling service providers and populating their stores with offers," CEO Bill Oesterle reassured investors. "We expect significant improvement in margin in the second half of the year and continue to see compelling opportunities for growth." When you couple the gale force competitive headwinds facing Angie's List with its still-lofty 30-plus forward P/E, I'd hold out for an even wider margin of safety before buying into that turnaround talk.