I expected to be able to glide on my boat into my 'harbor' which is a 3 x 4 square of half slabs (cobblestone) jutting out from my island base.

how ever, rowing onto the slabs, at any speed infflicts damage when the boat 'falls' the half block distance from the
top of the water to the half-block.

In snapshot 13w03a, this bug infflicted only about 3-5 hearts, seeming randomly.
But in snapshot 13w04a, this bug infflicts nearer to 8-9 hearts. (in both situations, i had protection 1 iron armor w/ diamond chestplate.)

How to reproduce:
1. find a body of water and layer a square of the top of the body with half-slabs. (so that they are on the same y coord as the top of the, say.. lake, but below the surface.
2. ride a boat onto the slabs.
3. feel the pain.