When Apple AAPL last week released a new operating system that permits ad-blocking extensions, all sorts of media publishers protested. But a much larger outcry may soon some from retailers and those who use their iPhones to make online purchases.

A Fortune investigation shows that an iPhone enabled with Crystal — the top paid iOS app right now – is unable to fully render the e-commerce sites of many major retailers, including Walmart, Sears and Lululemon.

The issue was first brought to our attention by Chris Mason, CEO of Branding Brand, a Pittsburgh-based company whose platform powers mobile commerce sites and apps.

“This upcoming holiday season… content-blockers are going to cause a lot of problems,” Mason says. “First, the experience for customers will be lessened. Lots of sites will be missing content, have broken links or customers won’t be able to add certain items to their shopping carts. They’ll probably just think the site is broken, but it’s really their content blocker. Second, retailers will be data-blind, or at least data-dark. It will really impact their ability to make quick judgments.”

Mason sent us a list of retailers with Crystal-related glitches, and we replicated them on our own iPhones. For example, check out this page for a pair of hunting boots on the Bass Pro Shops mobile website (as rendered on an iPhone 6 using iOS 9):

bass-crystaloff-520px

Now when the same page is loaded on the same device — but this time with Crystal enabled — the image of the boot disappears:

bass-crystalon-520px

But that’s only the tip of a giant iceberg that the USS Retail is hurtling toward.

For example, here is what happens when we went to Sears.com on mobile Safari without Crystal:

sears-crystaloff-520px

Now here is what happens when we went to Sears.com SHLD with Crystal:

sears-crystalon-520px

Notice anything missing? How about everything!

Walgreens.com WBA had a similar problem to Sears, when using Crystal. The homepage worked, but the Safari browser went blank after clicking the “Shop Products” link.

And, as Mason said, this issue goes far beyond just image rendering. For example, everything (mostly) loaded just fine on the mobile sites for Lululemon LULU and Walmart WMT with Crystal enabled. But it was impossible to add any products to the shopping cart. So if you just went to browse the pretty pictures, then there’s no problem. If you want to actually buy something, however…

Even for mobile websites that are working properly from a customer perspective, such ad-blocking technology also can strip out back-end code like Google Analytics or Adobe’s Omniture, which provide retailers with real-time insights into customer behavior. And then there is the whole matter of how retailers generate around 60% of their mobile web traffic inorganically, via online ads that Crystal and other ad-blockers are designed to eliminate.

“Retailers can work around it on the consumer side by doing a lot of recoding, but a lot of them freeze their codes on November 1, ahead of the holiday shopping season,” Branding Brand’s Mason says. “So that gives them just over a month or so to get it done. On the back-end they could use different sources of information for sales — kind of like checking the cash register instead of receipts — but it is a different process and also depends, in part, on if the sites are hosted on servers in-house or not.”

For retailers, this all presents a real and present danger. Even if only a small number of people so far have downloaded ad-blockers, there are two trends worth remembering: (1) The percentage of e-commerce being done on mobile is increasing; and (2) A disproportionate percentage of mobile purchases are made via iPhones rather than Android devices (which have allowed for ad-blocking apps for quite some time).

As for Crystal specifically, creator Dean Murphy said last night that he can remove select e-commerce sites from his app’s “blacklist,” and that he’d look into some of the examples we provided (four or five retailers already had contacted Murphy on their own, as of last night). In fact, several hours after we spoke, the Sears.com homepage was rendering properly with Crystal enabled, although we were unable to click through to many items. We also told Murphy about the Walmart shopping cart issue, and are now experiencing a similar problem as with Sears (i.e., product pages not loading at all). In short, these fixes seem to be tricky and ad hoc.

As for the back-end analytics, Murphy said that he was considering whether or not to create some sort of “tracking opt-out” functionality for users, but that he hadn’t yet made a final decision. He declined to say how many downloads Crystal has had, except that it topped 100,000 during a 12 hour promotional run in the App Store.

The trouble for retailers, of course, is that Crystal is just one ad-blocker. Another, Purify Blocker, currently sits at #5 in the App Store, and all of this is just one week after Apple unveiled its new operating system. Even if retailers reach out directly to one, they may be playing whack-a-mole. Moreover, they are entirely at the mercy of the ap

When media folks complained about ad blockers, we were called dinosaurs that had to change our business models. Does this mean, therefore, that retailers must abandon the mobile web? Or at least expect artificially deflated sales figures this holiday season? Perhaps. Or perhaps Apple will realize what it has wrought, and change its mind.