Assume you have a metal device box, correctly installed with 14/3 cable incoming and a three-way ungrounded light switch installed. Assume that the box is correctly grounded using the ground wire attached to the ground screw of the box.
No other connections exist in the box.
What should happen if one of the hot screw terminals gets close enough to the side of the box — say you were pushing the switch back into the box while it was energized and the side of the switch got close enough to the metal box to make contact?
Would you expect an arc? Would it trip the circuit breaker for that circuit back in the panel?