Mumbo Jumbo made this interesting scare machine a long time ago, but it's starting to be well known and people aren't falling for it anymore.
Therefore, there is now a variant "2.0".
If you press the button and already expect an armor rack to shoot towards you, you're wrong. The button simply activates a piston that is not attached to anything. The idea behind this is that you think you made a mistake while building, and then when you open the door to look, the armor rack comes shooting towards you first.
You can also just leave out the button....