Sometimes the back button just closes a sub-menu, sometimes it will close the whole menu. Often, the back button will drop me back at the main screen for the app instead of the place in the app I had come from. Sometimes I hit the back button by accident and there's no way to go "forward".

ETA since this happened to me: A notification will jump me into the middle of an app, like voicemail. Sometimes the back button will send me to the main page of the app instead of back where I was before the notification.