A Player cannot jump (parkour) into a 2 block gap from exactly .5 blocks lower
A player cannot jump (parkour) into a gap 2 blocks big, a block off the ground if they are standing on slabs. Similarly they cannot jump into the hole in between two slabs if on top of a normal block.

Short Video: http://www.dailymotion.com/video/x61g31w
Note: 0:00 - 1:00 in the video displays the bug.

1:01-1:33 in the video displays how if a player sprints and jumps at the last second the jump is still possible. I believe a player can make this jump by doing that because the player possibly falls slightly so they are not exactly .5 blocks lower, which makes the jump possible. (This fact that the player is able to make the jump by doing that might suggest that there is a bug with jumping, but this is mostly irrelevant)

What I expected to happen was...:
The player being able to jump into a gap that is 2 blocks tall from a y position .5 blocks lower.

What actually happened was...:
The player is not able to jump into the gap 2 blocks tall if they are .5 blocks lower. If they are 1 block lower, 1 block higher, or even at the same height they can make it through.

Steps to Reproduce:
A: Recreate the structure shown in the picture (don't forget the little trench) or the video (when the line is red) and

B:
1. Place two normal blocks (stone) on the ground right next to each other.
2. Place two blocks above the first two blocks with 2 blocks of air in between them.
3. On a side where two blocks are showing, place 4 slabs on the ground in a line for each block. Remove the slabs touching the blocks.
4. Attempt to jump into the gap between the blocks while starting on top of the slabs. This should be possible, but because of this bug it is not.
5. Replace the slabs with full blocks, remove the slabs, replace the slabs with 2 layers of snow, etc. (anything that changes the height) and try and jump through. Now the jump will be easily possible.