I have a never-rooted Nexus 4 running latest stock (android 4.4.4). It has been "hard-crashing" lately. By "hard-crash" I mean the screen goes black and either the device restarts itself (I see the same splash screen that is displayed during boot), or I have to hold the power button for a long time to get it to start (longer than for a normal boot).
After evidence pointing to it being caused by Google's own "Google Now Launcher" app (even after a factory reset), I began to wonder what it is about an apk file installed from the Play store on a non-rooted device that could ever cause the phone to "hard-crash". I would think that the sandbox in which apps run would prevent any app from being able to hard-crash a device.