World Boundary Problems

When i saw something on this website:

http://minecraft.gamepedia.com/World_boundary

It said I could pass the world boundary where it said "Illegal Position" in 1.8, 1.7, ect. I tried it with my friend and it still said "Illegal Position". I wanted to go pass it because it said there was almost no vegitation and i wanted to use it to build in a area with no plants and stuff in my way. why is there an "Illegal Position"?