By placing 2 by 2 dirt on top of 4 by 4 bedrock, placing 2 by 2 dark oak saplings, right-clicking the dark oak saplings with bonemeal until you grow a dark oak tree, chopping down the bottom of the dark oak tree above the dirt, placing 2 by 2 dark oak saplings on the dirt, right-clicking the dark oak saplings until another dark oak tree and repeating the same process will have a chance of the dark oak tree growing upside down and spreading roots through the bedrock and allowing griefers to pass through bedrock. This is a problem for adventure maps and servers that allow players to grow dark oak saplings and getting passed bedrock and causing problems. One way to fix this is to have dark oak saplings require 2 by 2 by 7 blocks of air above the saplings in order to grow the dark oak tree.