When you bud-power a piston, and place and delete a block on the extendable part (fast = in less then or equal to a gametick), the arm of the piston disappears. The only thing left is the block you placed (at the position it would have been shoved by the piston) and the base of the piston, which will retract as usual, but the head will not exist until it will be retracted again.

This could happen (theoretically) in both survival and creative, but since breaking things this fast is (until now) only achieved in creative, this is a creative-only problem.

This is tested in all directions, all of which are affected.

As far as I know this is fixable by checking the head of the piston together with the bottom when completely extended (after 2 RS-ticks, 4 gameticks).