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 department store operator Kohl's (KSS 2.69%) slumped on Thursday after the company reported mixed first-quarter earnings, falling short of analyst expectations for revenue. By noon, Kohl's stock had fallen by about 11.5%.

So what: Kohl's reported revenue of $4.12 billion, up 1.3% year-over-year, driven by 1.4% comparable-store sales growth. This fell short of the average analyst estimate for revenue by $70 million, and was modestly below the company's original expectations for the quarter, according to CEO Kevin Mansell.

Kohl's reported EPS of $0.63, up 5% year-over-year and eight cents better than what analysts were expecting. The earnings results were buoyed by a more balanced promotional environment compared to the same period last year, as well as strong expense control, according to Mansell.

Now what: Kohl's results weren't bad, but the stock was severely punished for missing sales estimates. The quarter was a vast improvement over the first quarter of 2014, when comparable-store sales fell by 3.4%, and Kohl's profitability was far better than what analysts expected.

The stock has run up quite a bit over the past year, rising about 33%, and the stock traded at about 17.5 times earnings before it tumbled today, so this extreme reaction may have been partially valuation-driven. Kohl's has struggled to grow over the past two years, and it has dramatically reduced its rate of opening new stores, so the stock may have gotten a bit ahead of itself.