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 many of Europe's major banks -- notably in the U.K. and Ireland -- were plunging today, with Allied Irish (NYSE: AIB) and Bank of Ireland (NYSE: IRE) both shedding more than 10%. Royal Bank of Scotland (NYSE: RBS), Barclays (NYSE: BCS), Deutsche Bank (NYSE: DB), ING (NYSE: ING), and Lloyds Banking Group (NYSE: LYG) were all close on their heels, with many close to double-digit percentage losses at their nadir.

So what: On Friday, I noted that Allied Irish's shares were surging after the bank was given a pass in the Euro region's stress tests and was a step closer to another capital injection. However, it's the former issue -- the stress tests -- that seems to be roiling the markets right now. Many investors are concerned that the tests weren't stringent enough and that European banks may be in even worse shape than feared.

Now what: Could this be a buying opportunity for investors with steely nerves? I'm wary of the banks in Ireland -- particularly Allied Irish -- because of the amount of capital that's been injected into them and the extent to which they are wrapped up in politics. In the U.K., though, I think the current valuation on Barclays could provide a decent buying point -- even if the future isn't particularly rosy. Meanwhile, my fellow Fool Anand Chokkavelu is torn -- he thinks the valuations look attractive, but, like many other investors, is wary of trusting the stress-test results.

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Fool contributor Matt Koppenheffer does not have a financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or Facebook. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.