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 low-cost wireless service provider MetroPCS (NYSE: PCS) plummeted 30% Tuesday after its quarterly results missed analyst expectations.

So what: Thanks to higher costs and lower-than-expected subscriber growth, MetroPCS posted second-quarter earnings of just $84.3 million, or $0.23 per share, versus the average analyst estimate of $0.28 per share. The shares have surged over the past year on strong revenue, profit, and new customer growth, so today's miss comes as an extra-big disappointment to Wall Street.

Now what: The weak economy, along with increased competition from the likes of Sprint Nextel (NYSE: S) and T-Mobile USA, should continue to weigh heavily on the company's subscriber count. In fact, rival Leap Wireless (Nasdaq: LEAP) is also down big today as investors run from the frightening low-end space altogether. Of course, when you consider that MetroPCS now trades at a forward P/E of roughly 8, the risks might finally be baked into the price.

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