When a piston (sticky or normal) is open you can go through the pipe-like object sticking out when you are near the base. The rest of the pile-like that can't be walked through has the same hitbox as a full block.
I have made this bug report private because this bug can be abused in adventure maps.
- duplicates
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MC-3004 Can walk through piston arm if piston placed facing east (f = 3)
- Resolved