Apollo 13 CSM malfunction manual

This Apollo 13 Command Module Service Malfunction Manual did not tell what to do in case one the spacecraft's oxygen tanks exploded during the space flight. But when faced with that occurrence, astronauts James Lovell, Jack Swiggert, and Fred Haise used the manual's cover to improvise a system that removed excess of carbon dioxide from the air, keeping them alive. Although not as expected, the manual did fulfil its purpose (thanks to its cover), helping the crew return to Earth safe and sound. This also explains why the cover is now missing - which promtply reveals a caricature on the first page of the manual.