What: Shares of business software specialist Tangoe (NASDAQ:TNGO) plunged Friday and were down 18% at 1 p.m. after its quarterly results and guidance missed Wall Street expectations.

So what: Tangoe shares have slumped sharply in recent months on concerns over sluggish growth, and poor Q3 results -- loss of $4.5 million on a revenue increase of just 3.9% -- coupled with downbeat full-year guidance only reinforces those worries. In fact, Tangoe's adjusted EBITDA margin for the quarter shrank to 5.5% versus 15.9% in the year-ago period, giving analysts plenty of negative vibes over the company's cost structure and competitive position going forward.

Now what: Management now sees full-year 2015 EPS of $0.34-$0.35 on revenue of $220 million-$221.3 million, well below the analyst consensus of $0.55 and $228.2 million, respectively. "While we continued to experience sales execution challenges and remain in a period of product transition, the company has made progress on taking the necessary steps to increase revenue over time," said CEO Al Subbloie. "We believe that Tangoe is in a position to grow market share longer-term due to the roll out of our new and enhanced Matrix platform." When you couple Tangoe's operational headaches with its particularly shaky competitive position, however, I wouldn't be too quick to bet on it.