3. Batman Begins
In the first of Christopher Nolan's superb Batman trilogy, Ducard and Crane are planning to spread a water-borne nerve toxin throughout Gotham City by vapourising the city's water supply with a microwave emitter stolen from Wayne Enterprises. An ingenious and creative plan, sure, but... The Human body is composed largely of water, so as soon as the toxin was released, absolutely everyone in Gotham would have exploded, or melted, or something, instead of just going mental at the gas in the air.