Hydrogen Powered Vehicles
The Hindenburg, a zeppelin created to showcase German engineering, exploded on landing in 1937.  35 people died gruesomely and the newsreel is still chilling to watch today.  Sparks ignited a flammable coating on fabric used for the airship's outer skin, the hydrogen inside exploded and the Hindenburg was, quite literally, history. 
It was designed to be filled with helium but the US government rightly refused to sell any to Hitler.  Today, can hydrogen possibly hope to recover from this famous PR disaster and become a true clean alternative fuel source for modern cars?