Bitcoin is a consensus system. It is paramount that all participating nodes reach the exact same state.
If undefined behavior could make some nodes accept a transaction while others reject it the state would become inconsistent and potentially mutually exclusive. From one currency, you'd have two and every coin could be spent twice.
If undefined behavior could make some nodes accept a transaction while others reject it the state would become inconsistent and potentially mutually exclusive. From one currency, you'd have two and every coin could be spent twice.