While shares of Adobe Systems (NASDAQ:ADBE) have generated outstanding returns for investors over the past few years, there are plenty of risks associated with the stock. A lofty valuation coupled with extremely aggressive growth targets creates a situation where even the slightest roadblock could cause the stock to fall. We don't know what will happen to the stock, but here are three things that could cause trouble for Adobe's stock over the next few years.

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Creative Cloud subscriber growth doesn't tell the whole story
While Adobe has been adding subscribers to its Creative Cloud service at an accelerating pace, all subscribers are not created equal. Adobe counts a customer as a subscriber regardless of whether they pay for the full creative suite, which costs $50 per month for individuals, or a single app subscription, which is far less expensive. The Photography subscription, which includes Photoshop, sells for just a fifth of the cost of the full suite, while a single-app subscription costs less than half.

The mix of Creative Cloud subscribers will be an important factor in determining how fast Adobe can grow its Creative Cloud revenue. During the fourth quarter, the average revenue per user for Creative Cloud, including all types of subscriptions, declined because of an increase in single-app subscriptions. In Adobe's perpetual license software business, single-apps represented about 50% of units sold, and Adobe expects the Creative Cloud subscriber mix to continue to shift toward single-app subscriptions going forward.

It takes five customers of the Photography subscription to equal a single subscriber of the full Creative Cloud in terms of revenue, and this makes the subscriber count that Adobe reports each quarter a little less meaningful. Adobe has set lofty revenue growth targets over the next few years, expecting its digital media business to grow at a 20% annual clip, but if Creative Cloud shifts toward single-app subscriptions more heavily than the company is anticipating, it could fall well short of its goal. Given Adobe's sky-high valuation, coming up short could be disastrous for the stock.

Competition in digital marketing
Adobe has emerged as one of the leaders in the digital marketing industry, and its Marketing Cloud product reported $1.17 billion of revenue in fiscal 2014. Adobe is expecting rapid growth going forward, calling for Marketing Cloud revenue to compound at a 25% annual rate through fiscal 2016, and IDC expects the market to grow by 50% through 2018. The growth opportunity for Adobe is very real, but so is the competition.

In the creative software business, Adobe's products are completely dominant, but the same can't be said about the digital marketing business. There are plenty of big competitors, including Salesforce.com, IBM, and Oracle, and all three will be going after the same customers as Adobe.

Salesforce made a huge acquisition in 2013, paying $2.5 billion for ExactTarget and seriously beefing up its own Marketing Cloud product. While Salesforce's main business is still customer relationship management, or CRM, digital marketing will be a big growth opportunity for the company.

Oracle and IBM are both in the digital marketing game as well, and they have plenty of resources to throw behind the effort. Oracle has made a string of acquisitions in the past few years, including the recent acquisition of Datalogix, the cost of which has been estimated to be in the hundreds of millions of dollars. And IBM, given its vast reach and existing enterprise customer relationships, will have a meaningful advantage as it competes in this growing market.

Adobe's 25% annual growth target may turn out to overly optimistic, given the growing competition in the space, and coming up short could spell trouble for the stock.

An outlandish valuation
Adobe's stock is priced with this rapid growth built-in, and anything short of perfection could send the stock plummeting. Even hitting the lofty growth targets set by the company may not be enough, given how optimistic the market has become.

Adobe currently trades at roughly nine times sales, 140 times earnings according to generally accepted accounting principles, 54 times non-GAAP earnings, and about 23 times Adobe's guidance for fiscal 2016 non-GAAP earnings. These numbers are staggering, but if the company can deliver the kind of earnings growth that it's calling for, they may be justified.

A big problem occurs if Adobe can't deliver on its guidance. It's the job of management to paint a rosy picture for investors, and it's the job of investors to determine how realistic that picture truly is. A lot can go wrong for Adobe in the next few years, and missing its growth targets, even by just a bit, could send the stock tumbling.