What happened

Shares of Comcast (CMCSA 1.57%) turned "red" on a "green" day for the stock market -- and Comcast has only itself to blame for it.

As of 1:11 p.m. ET, Comcast stock is down an even 5%.

Glowing red arrow trending down on a stock chart.

Image source: Getty Images.

So what

In a business update revealed at UBS' investor conference today, Comcast CEO Brian Roberts advised that the company will be looking at just 7% to 8% growth in earnings before interest, taxes, depreciation, and amortization (EBITDA) when it reports earnings next month, according to StreetInsider.com.

Granted, EBITDA isn't exactly the same thing as earnings calculated according to generally accepted accounting principles (GAAP), or non-GAAP earnings either, necessarily. Still, a prediction of no more than 8% EBITDA growth doesn't bode well for Comcast meeting analyst earnings targets of 28% growth for the quarter.  

Now what

Additionally, Comcast's CEO noted that the company expects to end the fiscal year with 1.3 million net broadband customer additions. On the face of it, that sounds positive -- but let's put that number in context.

In its earnings report back in October, Comcast told investors it had already added 1.1 million net broadband customers in the first nine months of the year. Thus, growth to 1.3 million in Q4 implies only about 200,000 new customers coming on board at Comcast -- a growth rate nearly 45% slower than what Comcast averaged over the first three quarters of the year.  

If you ask me, this is the number that spooked investors today. And coming on top of the prospect of an earnings miss, they may be right to be worried.