If the achievement does not pop, check your game settings. Achievements are typically disabled if you have enabled "Creative Mode" or used certain "Cheats" in that specific world. Ensure you are playing in Survival Mode with a clean save file. Additionally, make sure the piston actually makes contact with the mob or changes its movement path, as simply firing a piston near a mob may not count.
So go ahead—place that Glowing Carrot Cake, hide those sticky pistons, and watch as your first Chocolate Bunny takes a gentle plunge into your collection pit. Happy crafting, and stay lovely! lovely craft piston trap achievement guide new