To reproduce the fault:
Create a double piston extender pointing upwards (both pistons are sticky).
Place 2 blocks of gravel above the top layer of sticky pistons
Setup the redstone so that there a single input (lever or button - doesn't matter which) causes both pistons to extend/retract in quick succession.
Activate the input
Deactivate the input (lever/redstone torch/block...etc only)

What happens?
When the input signal is removed, either by lever-off or when a button ceases to provide power, the pistons retract and the gravel falls so that both gravel blocks occupy the same space immediately above the top retracted piston.

This is reproduceable almost every time, it seems to make no difference how long the piston is extended for as long as the double retraction is quick.

When re-powering the redstone both gravel re-appear in the expected 2 spaces above the extended pistons as if there had never been anything wrong.
causing the pistons to retract will cause the fault scenario to reoccur.

While the gravel blocks or co-habiting in the same space - it is not possible to interact with the blocks - you can not break them, add torches to the side or similar.

When using non-sticky pistons on the top layer this bug is not reproduceable - sticky pistons only.

when activating each layer of pistons separately with a level (or similar) the fault is not reproduceable.