1. You can place sand or gravel in the air, and it wont fall to the ground
2. if it falls down, it will jump around 0.5 blocks in one direction

What I expected to happen was...:
falling sand/gravel

What actually happened was...:
floating sand or wrong falling

Steps to Reproduce:
1. create a new World (creative, with cheats)
2. teleport to e.g. /tp @p 29999999 80 29999999
(other coordinates work aswell, but you have to be at one border of the world )
3. build a tower with dirt , and place some blocks on that
4.1. you will see, that some blocks fall down, and some will stay in the air (see: 2013-12-22_17.46.56.png)
4.2. some blocks will fall down, but they will fall at the wrong place (see: 2013-12-22_17.40.41.png; 2013-12-22_17.40.45.png) and on the ground, they will be placed back to the right place)