The story of an Air Canada plane which ran out of fuel in mid-air. The flight crew did the fuel calculation themselves on the back of an envelope when their computer broke down. Unfortunately, they confused imperial and metric measures and the calculated fuel requirement was too low. Running out of fuel in mid-air, the pilot managed to glide the plane down to a safe landing.
Could be used as a great example of why it's important to get your units right in every calculation! Leads into metric and imperial conversions.