Minecraft 14w17a, Minecraft 14w18b, Minecraft 14w19a, Minecraft 14w20a, Minecraft 14w20b
Windows 7 Pro
MC-54012 came after this, but was marked as duplicate for unknown reasons.
When you extend / retract a piston it shows the piston arm behind the piston.
Steps to reproduce:
1. Rapid pulse piston with clock
2. Notice piston head clipping out of piston itself