Gravity affected blocks such as sand and gravel, when they are placed in mid air, do not always start falling immediately.

Steps to reproduce:

1. Build a platform in mid air.
2. Place sand or gravel at the edge of the platform, while sneaking and also pressing the "move forward" button.

What I expect to happen:
The sand block immediately start falling before the sneaking player can stand on top of the unstable sand.

What actually happens:
Sometimes the floating sand stays there for a very short period of time, and the player will walk onto the sand block. Then the sand suddenly falls down taking the poor player with it.

Don't try this while filling in a lava lake if you don't want to lose all your things.