With a setup that has a piston (facing towards positive Y), with two or more falling blocks (e.g. Sand, Gravel, Concrete Powder) stacked directly atop it, when the power is switched on and off rapidly, one (or two in the case of the comparator/repeater clock shown below) of the blocks in the tower will be destroyed, rather than fall and be pushed back up as they should be.

Testing this showed that:

All falling blocks are affected

If the same is done with sticky pistons, it will yield the same result albeit you need to flip the power faster

Flicking power on and off while the piston is at rest in the "off" position, nothing of interest happens

Flicking the power on and off while the piston is at rest in the "on" position will break the block above the block that is directly on top of the piston

Doing this repeatedly will break all but one block in the tower

Sticky pistons and pistons blocks do not break the same block in the order of the tower under the same conditions

It appears to be due to when the two falling blocks separate, and the piston then reactivates as they fall, causing the blocks to get pushed into one another, the game then has two blocks in one space and decides to break one to remedy the problem

Using a clock for consistent pulse timing will break the same blocks each time they break

There is plenty more to be explored here, but for the sake of this not being a 40 page bug report I will stop there, at what I believe is the best possible description.

I have a screenshot of the setups that I used to test this, there is no redstone that is not visible on the screenshots (nothing is under the setups). Flipping the state of the comparator starts the timer.

The simplest way to reproduce this is as follows:

1. Place 1 piston facing positive Y
2. Place a lever next to it so that it will power the piston
3. Place falling blocks on top of the piston (I find that alternating the type of block helps in discerning which one broke.)
4. Flip the lever as fast as you can, and the tower breaks

I discovered this after using gravel as a substitute for me not having slimeballs for a sticky piston, so it is a bug that can be encountered during normal play.