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: Do you love roller coasters? Then you're going to love WellCare Health Plans (NYSE: WCG). The stock fell hard this morning -- down 19% -- before cutting its loss in half later in the day.

So what: Seems rival insurer Amerigroup (NYSE: AGP) discovered it had been double-billing the State of Georgia's Medicaid program on some of its bills. Upon being informed of this "anomaly," the state responded by initiating a search for similar patterns at insurers such as ... WellCare.

Now what: So far, there's no evidence whatsoever of monkey business at WellCare -- but investors aren't waiting around for even the first shoe to drop. They're fleeing WellCare in droves, and I can't say I blame them.

I mean, maybe there's no "there" there at all. But WellCare's already losing money on a trailing-12-months basis, and even if it hits this year's earnings targets, it will be priced at nearly 12 times current-year profits. That's not a lot of margin of safety for a dividend-less company that's only pegged for 13% growth as it is -- and could grow less if turns out to be infected by Amerigroup's "anomalies."

Will WellCare dodge the bullet that killed Amerigroup today? Add it to your Fool Watchlist and find out.

Fool contributor Rich Smith does not own (or short) shares of any company named above. The Motley Fool has a disclosure policy. Motley Fool newsletter services have recommended buying shares of Amerigroup. Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.