Apple’s latest mobile OS upgrade is causing older iPhones and iPads to crash when the owners try to upgrade to the new OS, according to a report from Reuters.

The report quotes one group of user saying that the iOS 9 upgrade would fail after several minutes, requiring them to restart the upgrade from scratch. Many of these users posted screen shots of the error message saying: ‘Software Update Failed’. Others reported that their device crashed when they tried to upgrade on a ‘Swipe to upgrade’ page.

The report quotes one iPhone user, Zorry Coates, who said she had spent three hours in the Apple Store in Sydney, Australia, and been given the option of returning the phone to factory settings, and losing any data that was not backed up in the process, or waiting until Apple announced a fix. “They said they were aware of the problem and their engineers were working on it 24/7, but they couldnt tell me when – or how – I would get a solution,” she is quoted as saying.

Earlier this week, Apple delayed the rollout of its updated Watch OS (OS 2), after it discovered a bug. Looks like the one in iOS 9 was somehow overlooked.

Sergio Galindo, general manager at GFI Software, said the problems are not unexpected given Apple’s decision to maintain support for older devices.

“Devices such as the iPhone 4s are significantly different and underpowered compared to more recent iterations,” he said. “This will have a bearing on compatibility and the ability to handle this new operating system build during installation and initial start-up after the upgrade. Apple’s own data shows that the iPhone 5s is over twice as powerful as the iPhone 4s, released just two iterations behind but which has received four major OS updates.”

Earlier this year GFI conducted research into which platforms had the most exploitable vulnerabilities. Apple with OS X and iOS was top, followed by the Linux kernel. iOS was found to have significantly more flaws than even conventional desktop and server Windows installations.

“Software glitches, vulnerabilities and compatibility issues in an embedded device such as a phone creates a challenging user experience,” continued Galindo. “There is an expectation that phones will always work as intended, built on our experiences with non-smartphone mobile phones which usually achieved just this. (Users) will not be prepared to accept handsets being rendered unusable or less usable for days or weeks while apps are updated and platform glitches are patched.

“This is why testing of new updates before allowing users to update their phones and tablets is essential. Applying to a business context, IT department needs to ensure users do not put the corporate network at risk. It’s better to wait until a patch is released before opening your gateway to the digital world.”

iOS 9 is currently trending on Twitter, though perhaps not for the reasons Apple hoped it would.