What I expected to happen was...:
that both pistons retracted but stayed on there place and that the block moved 1 block up (look at the pictures)

What actually happened was...:
pistons instantly double retracted and left the block without even moving it ones. this mean that the bottom piston failed to pull a block or that the upper piston pulled a extended piston. and that all within one single tick. (look at the pictures)

Steps to Reproduce:
there is really not much to it. just build the first picture you will need 2 sticky pistons, 2 blocks, 1 redstone and one lever.

pictures

if you will hit the lever this will happen (this is normal):

if you hit the lever again this happens (is a bug):