Unwanted Toast Notifications
Sounds like there's an app that is listening for the permission android.permission.RECEIVE_BOOT_COMPLETED and firing the toast much to your annoyance. 

You need to check with yourself by asking yourself those two vital questions - 


When did this happen? 
What was the last app you installed?


And backtrace it to the offending app. For an example, check out this and narrow it down to that aforementioned permission and see which one is it and un-install it.