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 electronics retailer Best Buy (NYSE: BBY) fell 10% today, on a number of factors because the future looks increasingly bleak for the big-box chain.

So what: With Best Buy set to deliver earnings next Tuesday, investors seem to already sense that the quarter was a stinker. Fellow big-box chain Sears (NASDAQ: SHLD) fell 19%, after it badly missed the mark on its earnings report last night, and Best Buy shareholders seemed unhappy with a five-point turnaround plan announced by CEO Hubert Joly earlier in the week. The new plan was mostly boilerplate, offering bland and recycled ideas.

Now what: The cries of "Mayday" may not have begun just yet, but this is a sinking ship. Poor management has bungled an already tenuous position as a brick-and-mortar chain in the sights of, and management still seems clueless about how to reinvent the brand and energize sales. Analysts are expecting a profit of just $0.12 a share, down from the $0.47 cents a year ago. If profits keep moving in that direction, it could soon be lights out for the superstore chain.

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