If you has been flashing your MIUI-supported Android devices to the latest 1.6.3 or 1.6.10, you probably found that some of the third party apps caused the phone to reboot automatically without any warning. No worry, you are not alone. It probably nothing to do with bad flash or bad download; it just the new permissions management system that is introduced in the new MIUI ROM.
In the new MIUI ROM, all the third party apps that require to access your SMS and Phone functions (and even ROOT) will be managed to prevent any malicious apps from accessing them. This permissions management system can be accessed from the System Tools, namely the Superuser. It is an app that manage your permission for the third party apps from accessing the SMS and Phone functions, and if the app does not have the permission, the phone will automatically reboot to prevent it from accessing them. As a result, you might be seeing some third party apps like Whatsapp cause the phone to reboot upon activating since it is trying to access the phone SMS function.