Last week, GT Advanced Technologies  (GTAT) made it abundantly clear that it feels it got a raw deal from Apple (AAPL), ultimately leading to its bankruptcy filing.

Specifically, GT Advanced Technologies —  which made the so-called sapphire glass used in certain parts of the iPhone 5 — accused Apple of employing a “bait and switch” tactic that put the company at more than its fair share of financial risk. The allegation was found in the company’s recently-unsealed bankruptcy documents.

According to GTAT, Apple told the company to “put on your big boy pants and accept the agreement.”

AAPL denies the allegations, of course, pointing out that GT Advanced Technologies agreed to the terms of the deal a year ago. GT seemed to have no problem with the supply contract then,  and willingly omitted any clause or condition that allowed for flexibility if unforeseen circumstances made it difficult to meet Apple’s requirements.

So which side of the table is the real victim? It’s an ugly reality, but this is an example of why no company should overpromise, overestimate,  or rely heavily on one (admittedly tough) customer.

Don’t Blame Apple

Yes, AAPL may well be the bully GT Advanced Technologies described in its recent bankruptcy filing. That can’t have been surprising to GT, though. Apple has been pushing its suppliers around for years now. Why? Because it can.

Unkind? Maybe. A little cold? Possibly. Then again, Apple has an obligation to secure the best materials at the best price it can, however it needs to do so, when it needs them.

Here’s the reality: If a small supplier can’t live up to its end of an agreement and/or doesn’t actually know how much it will cost to meet orders of significant size, then it shouldn’t enter an agreement. Customers rely on knowing they can get what they were told they could get when they need it.

A year ago, when the sapphire supply deal was being put together, GT Advanced Technologies said nothing about its concerns. Indeed, GT seemed thrilled and ready to do the deal, which called for $578 million worth of funding from Apple. As GT’s president and chief executive Tom Gutierrez said of the sapphire deal in a statement released on November 4th of last year, “We are very excited about this agreement with Apple, as it represents a significant milestone in GT’s long term diversification strategy.”

Sorry Tom, but the partnership with AAPL in no way added diversity. It all but committed your company to  one customer, and really just one product that you never actually had a prayer of delivering. It’s your product though, and it’s your manufacturing technology. You had a choice to walk away from the deal, ask for more time up-front, or tell your lawyers not to paint you into a corner.

But you didn’t — errantly assuming you could figure it all out before the deadline came.

Oh, and it’s not like AAPL is alone in using its size and clout. Most companies of that size can and do press suppliers or subcontractors to do more for less money, and ask for exclusivity. It’s called doing business.

In the same vein, too many companies have built an entire business on a foundation they had absolutely no control of.

Lesson (Not) Learned

It’s not as if relying on one customer for the bulk of your business is a newly discovered bad idea. We’ve seen several one-customer companies pay the price for a lack of diversity before.

Case in point: Zynga (ZNGA) recognized in 2011 that it was far too dependent on Facebook (FB) as a source of traffic. Indeed, some estimates suggested that 95% of Zynga’s business at the time was fed to it through Facebook. It didn’t do anything about it, however, and later that year ZNGA shareholders suffered for it when Facebook altered the agreement and was permitted to develop its own games.

ZNGA shares got cut in half in a matter of weeks following the news, in addition to the even-bigger drubbing the stock suffered in the months leading up to the revelation.

Second case in point:  In 2011 when Google (GOOG) deliberately changed its search algorithm to steer  browsers away from so-called spammy “content farms”, Demand Media (DMD) swung from a quarterly $1 million profit a year earlier to a $6.4 million loss in the same quarter Google’s search rules changed (and that was one of the modest setbacks — other websites really got hit hard).

Demand Media was simply too dependent on Google … a medium it had absolutely no control over.

And there are plenty of other ticking time bombs out there, albeit none as high profile as anything involving Apple. One of the more significant but underappreciated potential pitfalls of customer concentration is faced by EZchip Semiconductor (EZCH), which relies on Cisco (CSCO) for 39% of its business.

Cisco has already hinted at bringing this particular chip-making need in-house, but even if it doesn’t, when Cisco waves red flags of waning demand, EZCH shares get rocked.

There are certainly plenty more such case lurking out there, for those who dig deep enough.

The Bottom Line

There’s no denying the outcome of the GT Advanced Technologies partnership with AAPL isn’t what anybody wanted. But, just for the sake of discussion, what if Apple had been patient with GT Advanced Technologies or used a sapphire product that wasn’t up to the agreed-upon specs? The iPhone 6 launch would have been delayed and/or the product’s quality would have suffered. Had that happened, Apple would have been crucified.

In other words, accountability has to be accepted somewhere. Why not put it on the company that should have known it was overpromising and destined to not deliver?

And for all investors, keep this lesson in mind the next time one of the companies you own is betting the farm on one client or one product.

As of this writing, James Brumley did not hold a position in any of the aforementioned securities.