Let’s stipulate that only an idiot would panic over Apple’s third-quarter earnings “miss.” The company surpassed its own expectations for the quarter, and though it didn’t hit analysts’ predictions, the reasons it fell short are easy to understand.

As MG Siegler points out, Apple’s numbers are now largely a function of the release schedule of a single product—the iPhone. When Apple puts out a new iPhone, its numbers soar beyond all reason. As the phone gets older, people in the market for new phones reasonably decide to wait to see what Apple has in store for the next one—so Apple sells fewer and fewer iPhones throughout the year.

Analysts apparently did not anticipate how important this wait-it-out sentiment would be to Apple’s bottom line. In the previous quarter, Q2 2012, the company sold 35 million iPhones. Everyone knew Apple would sell fewer iPhones in the third quarter—according to Philip Elmer-DeWitt’s compilation, institutional analysts were expecting sales of around 29 million, while more bullish independent analysts thought we’d see 31 million. But sales were actually much, much lower than that. Apple rang up only 26 million iPhones in the third quarter, or only 74 percent what it sold in Q2. As far as I can tell, that decline represents the largest quarter-to-quarter fall in iPhone units in the product’s history. And the current quarter—Q4 2012—is likely to be even worse; the next iPhone is imminent, so even fewer people are going to decide to go with the iPhone 4S in remaining weeks before the next version.

But again—as Siegler and many others pointed out this week—this is all just a matter of timing. What Apple’s numbers show is that it has high seasonality, thanks to the fact that it now releases the iPhone in the holiday season (it used to release the phone in the summer). As long as Apple keeps hitting blockbuster holiday sales (as it did last year), it enjoys the freedom to grow only a little bit, or even slightly “miss,” during the rest of the year.

So it’s not time to panic. The sky isn’t falling. Apple is doing better than fine.

And yet, while I hate to be alarmist, I do wonder if it’s perhaps time to start thinking about considering to maybe begin preparing for the first unpleasant hint of something close to panic. Maybe the sky isn’t falling for Apple, but might the heavens be descending so imperceptibly that we all might be missing it?

Is that too vague? Here’s what I’m getting at: What Apple’s recent numbers underscore is that the company is high-wire act. Over the last few years, thanks to the iPhone and the iPad, Apple has been growing at a rate almost unprecedented in the history of large corporations. Nobody expects it to maintain that rate forever, and in fact—according to all metrics—Apple’s growth rate is now slowing.

This isn’t so bad in itself. But the challenge for Apple is making sure its growth rate declines gently rather than sharply. And that task becomes very difficult when you consider its dependence on the iPhone. Even to book steady rather than spectacular growth, every year Apple has to hit the iPhone way out of the park—and then beyond the parking lot and down the street and further still.

To put numbers on this, let’s look at iPhone sales over the past few years. In fiscal year 2009, Apple sold 20 million iPhones. In 2010 it sold almost double, 40 million. In 2011, it sold 72 million—80 percent more than the previous year. Fiscal 2012’s numbers aren’t in yet, because the year will end after the current quarter. But if we guess that the drop in iPhone units from Q3 to Q4 will match that from Q2 to Q3, then we can expect Apple to sell around 19 million phones this quarter. If that’s the case, Apple will have sold around 118 million iPhones over fiscal 2012. That will represent about 63 percent more than it sold in 2011.

See that pattern? Apple went from selling 100 percent more iPhones in 2010 to 80 percent more in 2011 to around 60 percent more this year. So let’s say that in 2013, a steadily growing Apple will need to sell about 40 percent more iPhones than it did in 2012.

What does a 40 percent growth in iPhone sales mean in real numbers? It means that after the next iPhone comes out, Apple will need to do something that sounds kind of crazy: Sell more than 51 million iPhones over the holidays.

Can it do that? I think it’s possible. But I also think it will be very hard to do. Understand that Apple has never cracked 40 million iPhones in a single quarter—last holiday season, it hit 37 million, its highest ever. To be sure, there is enormous pent-up demand for the new iPhone. But is there that much demand?

And let’s add even more wrinkles to this analyses: For one thing, it might be reasonable to consider that iPhone sales will get even more front-loaded next year, because there’s so much anticipation for the new phone that most people who want it will buy it more or less immediately. This means that over the course of fiscal 2013, we might see an even steeper quarter-to-quarter drop in iPhone sales than we’re seeing this year. If that’s the case, to maintain a 40 percent overall growth in iPhone sales for 2013, Apple’s holiday sales might need to greatly exceed a 40 percent year-over-year increase—meaning Apple might need to sell far greater than 50 million iPhone units in the quarter just to maintain steady growth.

And then there are macroeconomic forces pushing against Apple: European economies are in the toilet, the U.S. economy seems headed downward, and Apple’s growth in China appears to be slowing markedly. After expanding rapidly in recent years, the overall smartphone market appears to be slowing down, too. In the second quarter last year, global smartphone sales surged 77 percent over the year before, according to the firm Strategy Analytics. But this year they were up just 32 percent.

What’s more, Apple is facing ever-stiffer competition. As I’ve argued in the past, there was once a time when picking any phone other than the iPhone meant dooming yourself to a clearly inferior device. That’s no longer the case. Today, top-of-the-line phones that run Android and Windows are so good—and so competitive on price—that the iPhone no longer makes them look like a joke. I believe that most people will prefer the new iPhone to either of those two, but only fanboys can argue, anymore, than any one of these platforms ranks as head and shoulders above the others. For most ordinary consumers, the three phone platforms’ features will look roughly equal. And it will become increasingly difficult for Apple to stand so far ahead of the crowd.

The final thing to consider is something that seems so unlikely that I’m even wary of bringing it up. But it’s still something to think about: What if the next iPhone isn’t so great? What if it’s just OK—what if, after waiting for a redesign for over two years, people find the new phone to be not their cup of tea? What if they think it’s too similar to the current version—or, perhaps, that it’s too jarringly different? What if they find that it doesn’t have enough new features? Or what if there’s some kind of antenna-gate like flaw in it, a really consequential one this time?

Yes, this probably won’t happen. But remember that people don’t have to hate the new iPhone in order for Apple to have problems; it will be bad enough for Apple if people merely find the phone pretty good. Apple depends on the iPhone so greatly that it needs the world to fall head-over-heels for the thing. To sell 50 million units in three months, Apple will need to create something super-duper-mega wonderful. It’s certainly capable of doing so. But everyone makes mistakes.

Of course, there is one more thing to think about: The iPad. Apple is not a one-trick pony. It’s at least a two-trick pony, except one of its tricks—the iPhone—is a bring-down-the-house showstopper. But the iPad is coming along; each year, quarter after quarter, it becomes a more and more consequential part of Apple’s bottom line. In fact, it’s possible that in the current quarter, iPad sales could eclipse iPhone sales for the first time. (Apple sold 26 million iPhones last quarter and 17 million iPads; if iPad sales edge up because of the back-to-school rush, and if iPhone sales are depressed in anticipation of the next model, those numbers could well flip around this quarter.)

Now, more iPad sales don’t directly make up for the iPhone’s slowing growth, because the iPad is substantially less profitable than the iPhone. (That will be even truer if Apple puts out an iPad mini.) But if iPad sales explode over the next few years, that might not matter. The extra revenue will be enough of a cushion to keep Apple looking good even if the iPhone sometimes isn’t an unprecedented blockbuster. The iPad will pull Apple down from its dangerous high-wire act—and then we’ll all be able to take a breather.

[Flickr image by Martin Taylor.]