Blocks can be placed illegally outside of the world border by placing them on the outside facing face on a block that is inside the world border. That allows a 1 extra block range of placing that can be easily exploited by using smaller blocks i.e. fences or torches to place blocks in illegal spots.
Environment
Mac OSX 10.9 running minecraft 1.8 using java 8
Unfixable. MC-63578