I've developed a few iOS apps, and one of my favorite Apple "guidelines" (which they essentially enforced at the OS level without developer choice) was that, upon launch, your app should show a fake UI while doing startup tasks in the background. The recommendation was part of Apple's admonishment against splash screens. Think about how dumb this is, and how it makes your app look inept. Apparently plenty of developers did, and shunned this dumb idea; because Apple then forced it on developers whenever technically possible.
Upon your app going into the background or being kicked out of memory, Apple will take a screenshot of what your app is showing. When the user returns to your application, Apple will present this old screen shot; but none of the controls on it will work. The user can tap away furiously, but nothing will happen. When the app returns to functionality, the screen will be replaced by the real UI.
The problem here goes beyond ineptitude into a major privacy issue. You can think you "closed" or changed what an application is showing before handing your device to someone, only to find that Apple still shows a screen shot of its old contents in the open-apps stack. This could be a disaster.