The D-pad is too big for the screen on smaller devices and you can't really mine or do anything correctly.
This is how to get the bug:
1:Find an iphone 3, 4, 4s, or 5 or any device that is about the size of an iphone (like an ipod touch) and get minecraft PE on it.
2:Load up a new world
3:You can see that the d-pad is too big.