In 1935 the US Army tested a new bomber, the most advanced plane ever built. On the demonstration flight it climbed, stalled and crashed, and it killed the best pilot they had. The verdict at the time was that it was too much aeroplane for one man to fly.

They were half right. It was too much for one man to remember. The fix was not a better pilot or more training. It was a pilot's checklist, a short list of the things you do every single time. Then in 2009 a team ran the same test on surgery, across eight hospitals, and what they found changed how the best operating theatres in the world work.