""Unfortunately Google (or other app) has stopped error. I read recently about a privacy app that causes the above error to display to keep out unwanted intruders. Does anyone know this app? I remember something about opening yourself by tapping something 3 times or tapping and holding a button, just can't remember specifics. I know from many questions I read about the subject on here that it does happen for a legitimate error but I know I read about an app that uses it as a disguise, so to speak. Anyone?
One app that does that, is called App lock - fingerprint. 

This app has many cool features. One of them is, when someone opens a locked app, it will tell them that the app has crashed. They'll have to press and hold the ok button in order to have the keypad show up. (You have the option to enable or disable this option in the app settings)