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.
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Can confirm on 13w05a.
These are also affected (has same issue):
7-layered snow: 0.750
Enchantment Table:0.750
End portal frame: 0.813
8-layered snow:0.875 (same height as chests)
cacti: 0.938 (tested with 9-layered snow, 1.000 height)
And it seems that lilypad has similar problem.
On lilypad, player can place a block 2m above their head, so the player can stand in 1.984m gap.
But once goes out, he cannot enter there.
Confirmed