When trying to enter two block high doorways one block above you, it is possible to just walk through when standing on blocks that suspend you 0.5 blocks up (half slabs) but not on blocks that suspend you by 0.8 (End Portal frames and Soul Sand).
I listed this glitch under version 1.4.7, however i feel it is noteworthy that this bug has in fact been in the game for a long time.