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 specialty glass giant Corning (NYSE: GLW) plunged 11% on Tuesday after slashing its fourth-quarter profit forecast.

So what: Corning's cut was so nasty -- it now sees a 30% drop in earnings versus its initial guidance of just a 5% drop -- that analysts are being forced to seriously lower their valuation estimates. Management cited lower glass prices, a contract loss with a major South Korean customer, and weak demand for cover glass used in tablet computers as reasons for the revision, suggesting that Corning has plenty of trends working against it.

Now what: Buy-and-hold investors should look into this pullback as a possible entry point. "While we are clearly facing some headwinds this quarter, we are not changing our long-term growth expectations," the company said. "Our goal to grow company sales to $10 billion is still well on track with all of our segments growing." With Corning trading at a clear P/E discount to rivals like 3M (NYSE: MMM) and TE Connectivity (NYSE: TEL), going for that growth seems like a smart move.

Interested in more info on Corning? Add it to your watchlist.