Okay, so I've had this world since like 0.11.x, and when beta testing for 0.12.x (Nether was introduced) came around, I adopted my world as testing ground, and there was this one bug that REALLY "bugged" me. When I returned from the Nether random signs would just break. It seemed random, and I still haven't figured out the pattern (if there was ever one).
If you are unable to reproduce it in your world, try mine:
https://drive.google.com/file/d/0BxS2R36y8SarSjQ4V1JzWW5laFE/view?usp=sharing

-Device:
-Amazon's Kindle Fire HDX 8.9 (3rd Generation)
-Android 4.4.3
-MCPE 0.14.3