After reloading a world from the world-selection menu, sand placed on top of vertically-extended pistons appeared to have broken (the sand item entity was present while the placed block was not).

I experienced this problem while experimenting with double-piston extenders that extended two blocks of sand upwards, then retracted, allowing the sand to fall down two blocks and land on the pistons.
The world was saved and closed while both pistons of the double extender (bottom sticky, top regular) were powered and extended with two blocks of sand on top of the regular piston face.

Additionally, when reloading the world, the two pistons in the double-extender were both retracted (as if unpowered) despite both being powered (one by red stone torch on the side of an adjacent block, another by both a redstone dust line and a solid block with a powered repeater facing into it).

It should be noted that the second sand block (originally placed on top of the broken sand block) fell one block and landed in midair where the normal extended piston should have been (the piston was not extended despite being powered).

This problem persisted with gravel and red sand.