iOS Slide To Upgrade Bug: How Does This Happen?

In the first month after launch 90,000 different and unique device types were upgraded to Windows 10. At that sort of volume it's not surprising that there are a few problems along the way.
Apple released the iOS 9 update last week. It has to test on three device types (iPhone, iPad and iPod Touch) with perhaps half a dozen iterations of each. So how does it manage to let a bug which renders devices unusable slip through to release?
The Slide to Upgrade bug - as it has been christened - appears after iOS 9 has been installed and prevents the device from being used in any way, shape or form. The device proffers a screen asking the user to slide to upgrade after each reboot.
The fix is to connect your device to a PC or Mac running iTunes, create a back up of the device (if you don't have a recent backup) and then restore said backup to the phone. Not complex or particularly time consuming but potentially frustrating for end users nevertheless.
Is this symptomatic of a loss of quality control at Apple. It's hardly the first problem with the iOS 9 upgrade, not to mention the pulling of watchOS 2's release due to critical bugs. Apple needs to beef up its QA process (and resources) to ensure that its message of quality is reflected not just in the design of its hardware but also in its software releases.