On April 13, 1970—321,860 kilometers into its Moon trip—an oxygen tank exploded in the Odyssey's Service Module. James Lovell, Jack Swigert, and Fred Haise had a really big problem. These pages saved their lives.
These pages described how to power down the electric systems in the spacecraft, to save energy that was going to be needed during the final approach to Earth. After oxygen tank number two exploded and oxygen tank got damaged, they lost their ability to produce electricity using the service module fuel cells. which combined that oxygen and hydrogen to generate power.
It worked. The crew was able to power up their ship again with the limited power stored in Odyssey's batteries, and despite the fact that water condensation was everywhere after they turned the heaters off as part of their power saving measures. In April 17, 1970—18:07:41 UTC—the Apollo 13 crew splashed safely in the Pacific Ocean.
The pages—with notes from Lovell—will be up for auction April 13 at Bonhams in New York. [Bonhams and Wikipedia]