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.
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Duplicate of MC-108.