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 building-materials supplier USG (NYSE: USG) fell more than 12% in early trading on a 50% increase in volume. The stock closed off a little more than 10%.

So what: Not even a month after buying in, Big Money investors appear to be selling in the wake of a lousy third-quarter earnings report. Management cited low demand in explaining a $0.48-per-share miss two weeks ago.

Now what: I'm usually reluctant to bet against a Warren Buffett holding -- and Berkshire Hathaway (NYSE: BRK-A)(NYSE: BRK-B) continues to own more than 16% of USG's shares outstanding -- but there's little to like about the company's numbers or projections. Analysts see losses persisting through at least 2013, according to S&P Capital IQ. Do you agree with them? Would you buy shares of USG at current prices? Please weigh in using the comments box below.

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Fool contributor Tim Beyers is a member of the Motley Fool Rule Breakers stock-picking team. He owned shares of Berkshire Hathaway at the time of publication. Check out Tim's portfolio holdings and Foolish writings, or connect with him on Google+ or Twitter, where he goes by @milehighfool. You can also get his insights delivered directly to your RSS reader.

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