reproduce: build a "roof" that has a gap of 2 blocks from the floor and place a slab on the roof while standing under it and sneaking, aim 2 blocks infront of you on the closer end. The Crosshair isnt on the block that is selected. works with all the blocks i have tried.

basically the selected block and the Crosshair becomes becomes missallined so the block you are aiming at sometime isnt what the game thinks you are aiming at.