Guided Access is a handy feature in iOS 6 that lets you lock your child into a single app by disabling the hardware buttons on a device. It prevents your child from going hog wild through your iPhone or iPad when you hand it to them for some play time. Guided Access is great until it locks up and you can't exit from this mode.
In general, iOS devices don't freeze very often, but some unknown glitch causes Guided Access to lock up occasionally. When this happens, you are stuck in the kids app and can't get back to the home screen. I've had it happen with enough frequency that I've bookmarked this handy tip from OS X Daily that shows you how to exit out of a frozen Guided Access device.
When you are stuck in Guided Access mode, you are really stuck as the typical triple tap on the home button to quit out of Guided Access doesn't work. Even a force quit won't work because the power button and the home button are disabled. The only way to get out of Guided Access is to press the Home and Power button together for 15 seconds. This will turn off Guided Access by forcibly rebooting your device. Once your device is restarted, you can then go to Settings > Accessibility > Guided Access > and turn off Guided Access if needed. I usually keep Guided Access off by default and turn it on only when I am handing my iOS device to my children.
Apple iPad Air 2
Apple iPhone 6