If a chunk is unloaded while a piston is moving then there can be glitches regarding weather a piston is extended, not extended, or present.

if the chunk is unloaded at certain piston times then the piston may result in:

• visible extended piston head, no visible piston block
• piston block with both extended and unextended head present

for a piston with both extended and un-extended head then breaking and replacing the piston can repair the error.

for a piston with a visible extended head and no visible piston block, breaking the head will remove only the visible head with no drop.  The block will still be present but can not be seen or interacted with.  It is as if the map says "This block space is taken" but has a null for what is occupying that block space.  Only TNT or other explosive charge can remove the invisible block.

server/map reload or client reload does not repair the block issue.