The iPhone Duo, one of Apple Inc.’s biggest new products in years, is drawing a growing number of complaints from customers who say the foldable display cracked after just hours or days of use.

In these cases, customers reported hearing a crunching sound when unfolding the phone, followed by a large crack forming along the center crease. Some users also experienced a white patch or other display malfunction on the right side of the screen.

Reports of the problem first surfaced on Reddit and other social media sites. Bloomberg News has since independently verified the issue with more than half a dozen customers, who provided photos showing damage to their devices or evidence of their support conversations with Apple.

Apple acknowledged the issue in a statement, saying it was “aware of an issue impacting the inner display on a small number of iPhone Duo units.”

“If users experience issues with their iPhone Duo display, they should contact AppleCare for a replacement,” the Cupertino, California-based company said. Most Duos aren’t affected by the problem, and the units that are impacted exhibit the defect quickly, Apple said.

“I went to unfold it and it felt stuck/harder/slower to open than usual. And when I opened it it had the lines on the screen,” Tom Battle, an Apple customer from San Diego, told Bloomberg. “You could feel a bump or loose piece under the display.”

Venture capitalist Jonathan King had a similar experience, saying he heard a crunch-like sound when opening his device on Monday.

“I had all intent to post a positive review of my Duo experience today, until it happened to me,” King said. “The smaller outer screen works just fine, but the inner screen is now unusable,” he said, adding that the device thinks he is touching the inner screen when he is not.

Read More: Apple’s Most Exciting Phone in Years Has Growing Pains

While the problem appears to affect only a subset of units, it represents an embarrassing setback for Apple, which spent more than a decade developing the device and touted its superior durability, hinge design and display technology.

The Duo, which went on sale Friday, represents the company’s first foray into foldable phones — and the biggest-ever design change to the iPhone, Apple’s largest source of revenue. Like other foldable phones, the device has a smaller screen on its outer surface and can be opened to reveal a larger display.

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Chief Executive Officer John Ternus previously dismissed competing Android foldables as “cheap and crappy,” while the company’s head of hardware engineering, Tom Marieb, encouraged customers to hold Apple “accountable” for the screen crease’s durability. During the phone’s development, Apple worked to reduce the visibility of the crease in the middle of the opened display — a problem that has plagued previous foldables.

Bloomberg didn’t encounter the issue on one review unit provided by Apple or another device purchased independently.

“I was reading Apple News and decided to open the display to enjoy it, and that’s when I saw the crack line and bruising,” a user who goes by Centrino31A told Bloomberg. “There is no damage to the outside of the phone nor to the inner display surface. It is pristine. The issue is the underlying display.”

The customer said his local Apple retail store in Richmond, Virginia, called the issue a defect that required a replacement, but the location had no Genius Bar customer-support appointments available. Apple then sent him to another store, which conducted testing to confirm there was no accidental damage. But that location had no inventory, so he is still waiting for a replacement device.

Centrino31A said he understands that first-generation devices can have unexpected issues, but that it was “extremely frustrating” that stores didn’t have separate inventory for addressing manufacturing problems, particularly for a device that starts at $1,999.

In all cases, users said they felt a bump under the display near where the crack started. Repair platform iFixit, which disassembles products to study their components, said it believes the issue is caused by a piece of the hinge failing and cracking the screen, rather than a problem with the display panel itself.

The company has faced issues with new iPhone hardware in the past, including the iPhone 6 Plus in 2014, which would bend easily, and the iPhone 4 in 2010, which sometimes lost reception when users touched the antenna cutout around the device. In both cases, the company emerged from the problems relatively unscathed.

The latest difficulties show that “even a company with Apple’s engineering resources and supply chain discipline faces a steep learning curve in foldables,” according to IDC analyst Francisco Jeronimo.

Samsung Electronics Co., which pioneered the foldable field, also had problems with its first Galaxy Fold in 2019, Jeronimo noted. “It also showed that a fast and transparent response can contain the damage to a category,” he said, adding that Apple’s move Monday to send users to customer care is “the right first step.”

Vishnu Rajkumar, an Apple customer who said he has used previous foldable phones from OnePlus and Samsung, also experienced the Duo issue. “I was setting up my apps and wanted to check how Outlook works in the wider inner screen,” he said. “I tried opening the screen and found an unusual tightness.”

After applying a bit more pressure, “the screen started bleeding,” he said. “Initially the line was small, and it increased further after multiple folds.” Rajkumar, who lives in Chennai, India, was able to locate a replacement device at an authorized reseller and said that model hasn’t had any issues during his first 24 hours of use.

Read More: Apple’s iPhone Duo Hits Stores in Face of Short Supply and High Demand

Another user, Surya Mangi of Allen, Texas, said the inner screen cracked after about 30 hours of ownership during normal opening of the display. “Trying to replace but no Apple Store around so asking me to send in and be phoneless for over a week,” he said. Apple is ultimately shipping Mangi a replacement by express mail.

Apple employees said the company hasn’t provided any official guidance to its retail stores or customer service agents about the problem as of the initial publication of this story. But a team is investigating the issue. In some cases, Apple engineers are trying to obtain affected devices for analysis.

One customer, Justin Lokos, said Apple did what is known as an Early Field Failure Analysis on his device. “I gave as much detail as possible,” he said in a note. “All of the employees in store were very intrigued (imagine I was the first person there to have this issue).” He received a replacement at Apple’s Fifth Avenue store in New York.

Lokos said he was sitting on his living room couch at 5:40 p.m. using the iPhone Duo to order Grubhub, but the experience wasn’t ideal on the outer screen because the app isn’t yet optimized for the new design. He thought it would work better unfolded, but when he opened the device, he heard a crunching sound and the display cracked.

Another user, Jainam Shah, said his iPhone Duo’s hinge felt perfectly normal until one particular unfolding, when it suddenly became noticeably stiffer. “I continued opening it, heard an audible crack and immediately noticed the bright vertical and horizontal lines across the inner display.”

Shah said there had been no drops, impacts or unusual pressure on the phone. “Took it straight back to the Apple Store, where the employees were just as surprised,” he said. “They replaced it immediately and asked if I’d be comfortable having engineering reach out to understand what happened.”