Month ago my Lumia 520 fell on concrete from about 1 meter height. Luckily the screen and its frame took the hit resulting in a crack in the bottom right corner which broke 50% of the touch screen.
Today I replaced the touch screen and frame, confirmed that it was the only thing that had damage on it and booted the phone on. Everything was and is working fine.
Except the precision in the bottom is not great and most of the time it's bad right in that right bottom corner where the crack was. What's weird about it is that it works perfectly (just like it should) when the phone is in landscape mode. Also, while writing this question, I discovered that when the phone is not in the hand, on the desk, for example, its back/start/search buttons aren't working.
I have checked the touch screen cable, reseated it, wiggled it and everything else and nothing. It does not matter what touch sensitivity is set. Currently have installed Windows 8.1 altough the same issue was present on the latest Windows 10 Insider Build.