A meter is longer than a yard. An ounce is heavier than a gram. We harmlessly mix them up sometimes, but a "unit conversion error" when you're filling up the fuel tanks of an airliner can be fatal. Which is exactly what happened to Air Canada Flight 143. 

Tim Harford talks to mathematician and comedian Matt Parker about how the aircraft came to take off without the proper fuel load, how no one noticed until it was too late, and why such errors give us an insight into just how important maths is to keeping our complex world working as it should. 

For more "unit conversion errors"  check out Matt's book Humble Pi.