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 online photo expert Shutterfly (Nasdaq: SFLY) were in a tailspin today, falling as much as 17% in intraday trading after Apple (Nasdaq: AAPL) revealed that it's getting into the company's business.

So what: It wasn't much of an announcement, but during its presentation of the new iPhone, Apple revealed a new app that will let users design greeting cards on the iPhone or iPad and then have them printed and mailed by Apple to addresses all over the world. With Apple seemingly unstoppable right now, if there's one thing you don't want, it's the “king of i” becoming a direct competitor.

Now what: Obviously Shutterfly doesn't shrivel up and die simply because of this app, but this definitely gives Shutterfly shareholders something to think about. Growth expectations are high for the company and investors are currently paying a price that bets heavily on that growth coming to pass. In the coming quarters we'll see in black and white whether Apple actually makes a meaningful dent in Shutterfly's business, but from where we stand today, I'd definitely be wary.

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This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.