Imagine it's the early summer of 2007 and you're in an Apple conference room playing with an iPhone prototype. The iPhone will be released to the public in a few weeks. It's a cool device, but you -- and everyone else in the room, except maybe Steve Jobs -- has one major concern.

There is no physical keyboard. Unlike a Blackberry, users will need to type directly on the screen. 

According to the new documentary The iPhone at 15: An Inside Look at How Apple Transformed a Generation, Apple execs were also worried the device might fall flat.

(In an all-time top five opinion that didn't age well, erstwhile Microsoft CEO Steve Ballmer said of his first reaction to the iPhone, "$500? With a plan? If that isn't the most expensive phone in the world. And it doesn't appeal to business guys because it doesn't have a keyboard. Which makes it not a very good email machine.")

Then Apple execs noticed how "sticky" the new phones were. Testers couldn't put them down. Typing on a screen? No big deal; plus, the lack of a physical keyboard left more screen real estate for viewing photos, videos, etc. As for email? Clearly the iPhone turned out to be a very good "email machine."

So maybe, if you were in that conference room in 2007, you saw the success of the iPhone coming.

But here are a few things the documentary shows Apple execs didn't see coming.

The Popularity of the App Store

The iPhone 3G was launched in 2008. So was the App Store, which allowed outside developers -- from one-person startups to large firms -- to reach users and customers. In hindsight its success seems obvious, but at the time Apple execs felt that receiving 50 submissions would be "a nice little start."

They received 500.

By April 2009, 25,000 apps were submitted for approval every week. As one longtime developer says, "The App Store has developed into the richest, most diverse, and most accessible software ecosystem the world has ever seen."

A little over the top? Maybe. Unless you consider unicorns like Instagram, Waze, Uber, Snapchat...all of which started out as iPhone apps.

The Need for a "Mylie Rule"

One day App Store review executive Phillip Shoemaker saw a $450 charge on his credit card bill. The payee? Apple.

Turns out his daughter Mylie had made in-game purchases of Smurfberries while playing "Smurfs' Village."  

Shoemaker wasn't the only person concerned; Apple received a number of customer complaints about hidden -- or too seamless -- in-app purchases. The result was Mylie's Rule, an iOS update requiring a password for any purchase made while using an app.

The Popularity of the Second Camera

Photography was always a major point of focus. In fact, Apple considered making its own cameras, since it already knew more about photographic hardware and software than almost any other company in the world. But then execs realized that no one wanted to carry two devices.

"Why would you carry a second camera," says Apple SVP of worldwide marketing Greg Joswiak, "if your phone is the device you're going always carry with you?"

So Apple focused on continual camera and software development.

That included adding a front-facing camera to the next phone, the iPhone 4G, largely to facilitate FaceTime. "We are trying to figure out," Apple engineering manager Justin Santamaria says, "what do you do with the front-facing camera?"

Turns out: a lot. The front-facing camera ushered in the selfie era. And apps that used the front-facing camera, like Snapchat.

And speaking of FaceTime, it was one of the first apps to use Apple's push-notification sounds and pop-ups. At the time, execs thought notifications would streamline the iPhone experience.

Nope: Notifications demand more attention, not less.

The Demand for Larger Screens

For years, Apple felt screen size should remain constant.

Then, in 2014, Android screen sizes expanded to between 5 and 6 inches. Apple successfully sued Samsung for patent infringement -- unrelated to screen size -- and won a $530 million judgment.

But in the process, Apple execs realized their own screens needed to be larger. The Apple 6 and 6 Plus featured larger screens and became some of Apple's best-selling devices. 

What Users Would Actually Do With Their Phones

Andy Warhol once said, "Don't think about making art, just get it done. Let everyone else decide if it's good or bad, whether they love it or hate it. While they are deciding, make even more art."

Steve Jobs -- and by extension, the people at Apple -- took a similar approach. 

As Jobs said

I'm a tool builder. That's how I think of myself. I want to build really good tools that I know in my gut and my heart will be valuable. And then whatever happens is...you can't really predict exactly what will happen, but you can feel the direction that we're going. And that's about as close as you can get.

Then you just stand back and get out of the way, and these things take on a life of their own.

Or as documentarian Joanna Stern says, "The thought these people put into the iPhone was more about making it the best device possible, not as much what world-altering impact it would have."

Which is all you can do. Create the best products you can. Provide the best services you can.

That's all you can do.

Your customers will decide whether -- and how -- what you provide meets their needs. Solves their problems. Makes their lives better. 

And in the process, they may find uses you never predicted.