It is sometimes not possible for a player to place a block in the same block space as they are standing, even if such a placement wouldn't cause the player to be inside the block.

This can be tested by placing a fence block, walking up to the fence until touching it, breaking the fence, and trying to place it back in its position, however this is quite inconsistent. A better version is as follows:

• Place a 2x2 square of fences, glass panes or iron bars

• Position yourself inside this square

• Break one of its constituent blocks

• Attempt to place the block back where it was

It isn't possible to place the block while between the three other blocks, even though placing the block wouldn't have the player clip through it.

A similar effect can be seen using snow layers:

• Place down one snow layer

• Stand inside it

• Attempt to place more snow layers

The placement will fail even though snow layers of heights 2 and 3 do not collide with the player and as such should be placed with no problem.