At the very end of the reach of the player, block outlines don't show up anymore, but it is still possible to mine and place blocks. Interacting with them makes the GUI show for just a few frames and then it disappears again.

An easy setup to reproduce this:

• Place a head against a wall 

• Walk into the head

• Turn around and look at the floor beneath. Slowly move the camera upwards. 

• As soon as the outlines disappear, place or break a block.

(visualization of the entire thing attached)