Water will flow under falling sand for a short amount of time.
What I expected to happen was...:
I didn't expect the water to flow under the falling sand, it did not happen in 1.4.2.
What actually happened was...:
Water was flowing next to a powered piston with a sand block on top and, when unpowered, the water flowed under the sand before it fell.
Steps to Reproduce:
1. Place a regular piston facing upwards on the ground.
2. Place sand on top of the piston
3. Place a redstone torch on the block under the piston
4. Place a repeater to power the block with the torch on it.
5. Add a line of redstone to power the repeater.
6. Add an enclosed water source block next to the extended arm of the piston.
7. Place a redstone torch at the end of the line of redstone to unpower the piston.
8. The water flows under the falling sand.
Caused by the sand generator dupe fix, which added another tick before sand starts falling.