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: Whoo! That was close! After an initial steep sell-off, the Dow Jones Industrial Average (INDEX: ^DJI) -- and other indices as well -- somehow managed to end the day in the green. Sharing in the relief were shareholders of residential mortgage REIT American Capital Agency (Nasdaq: AGNC), which dove as low as 12% down before ending the day (slightly) in the green.

So what: American Capital is one of several mortgage REITs, including investor favorite Annaly Capital (NYSE: NLY), that have been pummeled in this latest market selloff. But American Capital has been singled out for comments made by its president, Gary Cain, today, to the effect that Europe's banking crisis is putting "potential pressure" on the rates it must pay (and the profits it hopes to earn) when seeking short-term "repo" financing for its business.

Now what: One-month repo financing, as of this morning, cost 25 basis points on the amount of the loan -- up 25% from the rate in effect on Aug. 31. That's not a lot of money, admittedly -- 25 "basis points" means one quarter of 1%. But it's a pretty big jump in just one month's time, and it highlights the volatility of this market, making American Capital's profits less certain than investors might like.

With the stock pegged for nearly no growth for the next five years, any volatility at all could be enough to destroy the growth thesis here. If this possibility makes investors nervous, well, perhaps they're right to be nervous.

Want to learn more about American Capital Agency? Add it to your Fool Watchlist.

Fool contributor Rich Smith does not own (or short) shares of any company named above. The Motley Fool owns shares of Annaly Capital Management. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.