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 NII Holdings (NASDAQ: NIHD), the mobile phone carrier that offers service under the Nextel brand in Brazil, plunged 10% today after the company's quarterly results missed Wall Street expectations.

So what: NII shares have been slaughtered over the past year as the rollout of its 3G network has struggled to gain traction, and today's third-quarter results -- per-share loss of $0.48 from a breakeven quarter in the year-ago period while revenue fell 15% -- suggest that things are only getting worse. In fact, the company's Brazilian subscriber base fell by 92,000 customers during the quarter, reinforcing serious concerns over intensifying competition and its seemingly out-of-date technology.

Now what: Moving forward, management will continue to eliminate expensive and difficult-to-retain customers in Brazil. "While this plan will mean that churn will rise substantially for the fourth quarter of 2012, causing us to fall well below our consolidated net addition goals for the year, it will position Nextel Brazil to return to positive net add growth in the first quarter of 2013, with a higher-quality subscriber base," CEO Steve Dussek reassured investors. "Executing this strategy is the right decision for the business in the long term." Given just how rapidly NII's expenses, cash burn, and debt load are growing, however, the short-term risks are just too serious for average Fools to take on.

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