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: J.C. Penney (NYSE:JCP) shares were down double digits for the second time in a week today, falling as much as 11% after one of its biggest shareholders sold off a large stake in the company.

So what: Vornado Realty Trust, J.C. Penney's second-largest shareholder, let go of 10 million shares of Penney's stock, a majority of its stake and nearly 5% of the retailer's total shares outstanding, for a price of $16.40 yesterday. The move is significant not just because a major investor is unloading so many shares, but because Vornado's Chairman, Steven Roth, sits on J.C. Penney's board and was once a major defender of the company.

Now what: Vornado's sale comes just a week after Penney's absolutely atrocious fourth-quarter report, which included a 31% drop in same-store sales, among other equally disconcerting results. It would be hard for most retailers to see that kind of drop if they tried to lose customers. J.C. Penney shares hit a four-year low today, and after a year of nothing but bad news, the bull thesis is becoming less and less tenable. Some analysts believe that CEO Ron Johnson may have only six months left to prove his turnaround strategy.

For more on J.C. Penney, I recommend picking up a copy of our premium research report all about the department-store chain. It comes with in-depth analysis on the company's opportunities and risks, and a year's worth of free updates as a bonus. To get started with this top-notch new package, just click right here.