Minecraft 1.4.3, Minecraft 1.4.5, Minecraft 1.4.6, Minecraft 1.4.7, Snapshot 13w05a, Minecraft 1.5, Minecraft 1.7.4, Minecraft 14w03b
It is known, that you can prevent people from entering a 2 high gateway by placing soulsand in front of it. But in 1.4.3 i found it to very inconsistent.
You can actually enter the gate by walking diagonally into it. BUT only into gates facing North/South or East/West. (Didn't know how to find out. But it only works on one of them)
The same now works with chests and their new hitbox. Soulsand and chests are buggy.
I hope the screenshots clear it up. Else i attached the worldfile, so you can try it yourself.