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 mobile device accessory maker ZAGG (Nasdaq: ZAGG) are sagging heavily today, down by as much as 15% after the company increased its full-year guidance.

So what: The company, which makes the popular invisibleSHIELD protective screen, expects full-year 2011 sales to top $175 million. This beats its previously guided range of $170 million for the year, and also bests the $171.8 million consensus estimate. The company attributed the bump up to strong holiday sales for both ZAGG and iFrogz, which ZAGG acquired earlier this year.

Now what: It also saw strong online sales in the fourth quarter, and expanded its product offerings and retail presence. Sometimes the market makes you scratch your head, and this move is one of those times. It's clear that investors were evidently looking for more, but the announcement is far from disappointing. ZAGG has been zig-zagging all year while putting up solid growth. If you've been on the fence about picking up shares, today is your chance.

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Fool contributor Evan Niu holds no position in any company mentioned. Click here to see his holdings and a short bio. Try any of our Foolish newsletter services free for 30 days. We Fools may not 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.