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.
There are two methods to prevent the reboot issue from happening again (though I strongly recommend the first):
Give the permission of accessing the SMS and Phone function to the trusted third party apps
How? Enter the Superuser app, then all the apps asking for the permission to access SMS and Phone functions should be listed here. Click on the app that you want to modify, then click on the permission type. For instance, Whatsapp wanted to access both SMS and Phone functions, so click on both permissions, and set them to Allow (not Denied or Always ask as they will reboot your device). Problem solved!
Turn off the Superuser from managing the permissions to access Phone and SMS functions
How? Enter the Superuser app, then check the bottom of the screen, you should see a slider for Manage Permissions. Slide it from the right to the left to turn it off, then click OK for the warning appeared. Problem solved, although I do not recommend this.
It’s always great to see that the more and more custom Android ROM community are taking the apps permission seriously (which I personally think it should be there in Stock ROM), and making them even better than the default Android ROM. Hope the above methods are able to help in solving the issue you might be having with MIUI 1.6.