People shouldn't stab their pets.
Bob the magician wants to practice his knife through the box illusion but no one will practice with him.
Bob wants to test the trick to ensure it's safe.
Bob fills the box with balloons and jams knives, swords, etc into the box.
Bob puts his dog in the box and jams knives, swords, etc into the box.
Some of the balloons pop and Bob knows where in the box the issue is.
...turns out it wasn't safe after all, as the test was failed.