After breaking a block below a sand (or gravel) block, the falling sand animation begins one block above where the sand block was previously. When a different block is pushed into the space created before the sand starts falling, it destroys or displaces the falling sand. In version 1.4.7 it would be placed with no problems, and the stack would remain the same height. This makes it no longer possible to add to a stack of sand/gravel after it is already falling, rendering some redstone devices useless.

To replicate this, make a stack of sand, then make a piston that is connected to a repeating redstone signal, and have it push a block into the stack. This is shown in the picture below, with the obsidian being where you'd place a new block.