When opening a world you spawn in the air.

Samsung Galaxy s4

When you open a world, you end up spawning a block in the air which has a couple disadvantages that I can think of: if you are in a 2 block high space when you open the world, you spawn partially in the ceiling causing a half heart of damage; if you are on crops when you start the game, you end up trampling the crop, cause you spawn in air and fall on it.