When a piston pushes up a redstone block, you can not retract this piston as long as the redstone block is up there.

How to reproduce:
Step 1: Place an upwards facing piston.
Step 2: Place a redstone block on top of it.
Step 3: Choose your favourite way to power the piston.
Step 4: Unpower the piston.

EDIT: And no, this is not a duplicate of MC-108. MC-108 will not be fixed, but this is, as far as I can tell, unwanted behaviour as a result of MC-108.