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 O'Reilly Automotive (Nasdaq: ORLY) sank 18% on Wednesday after the auto parts retailer warned that its second-quarter results would come in below Wall Street expectations.

So what: O'Reilly shares have soared over the past year on strong earnings momentum, but management's disappointing guidance -- EPS on the low end of its prior forecast and lower same-store sales for the current quarter -- is forcing Mr. Market to sober up. In fact, the news seems to be triggering concerns over slowing growth in the do-it-yourself auto market space, sending stocks like Advance Auto Parts (NYSE: AAP) and AutoZone (NYSE: AZO) down today as well.

Now what: For the second quarter, management expects EPS in the lower end of its previously announced range of $1.13 to $1.17 and same-store sales growth of 2% to 2.5%, down from a prior view of 3% to 5%. "We saw improved comparable store sales results for the month of May; however, comparable store sales in June were below our expectations," CEO Greg Henslee said in statement. With the stock still up more than 40% from its 52-week low and trading at a not-so-cheapish P/E of 20, however, I'd wait until more of that risk is baked into O'Reilly before jumping in.

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