On April 13, 1970, 36 years ago today (yes, it is still April 13th in my part of the world  ), a small list of tasks was sent up to the crew of Apollo 13; a mission ment to be the 3rd manned lunar landing.  On this list of tasks included the common "housekeeping" procedure to stir the oxygen tanks.  These tanks were the source of electrical power for the spacecraft and they had to be stirred with electric fans to keep liquid from pooling.  

When mission crewmember John L. Swigert hit the switch to acivate the fans in the oxygen tank, there was an explosion.  Seconds later, mission commander James Lovell uttered the famous words "Houston, we've had a problem."  (Not: "Houston, we have a problem").





The crew safely returned to Earth on April 17, 1970.

Following the mission, the investigation found the cause of the explosion.  A few years before the mission flew it was decided that starting with Apollo 13 these oxygen tanks would be upgraded, the previous plan had been to use the upgraded tanks starting with Apollo 16.  The tanks were removed from the Apollo 16 spacecraft and placed into the Apollo 13 craft (Apollo 16's were replaced once more were made) but in the process one was dropped a few inches.  It was inspected and shown to be undamaged.

A few years later, only days before the launch the rocket was rolled out on the pad and prepaired for launch.  Part of the preparation included pressurizing these tanks by heating them to a specific temperature.  During this process, the dropped tank heated up far above where it should have due to a damaged theromstat sensor (damaged in the drop and overlooked), this overheating went unnoticed as well.  Due to the overheating, protective coating on the electircal wires for the stirring fan was damaged.  During the mission, the fan worked normally and was used to stir the tank a couple times.  But when it was needed to stir it on April 13th, this time there was a small spark due to the exposed wiring in the tank.  In the pure oxygen environment of the tank, an uncontrolled combustion of the gas occured instantly causing an explosion that blew a panel off the spacecraft and damaged nearby tanks.  

This resulted in the spacecraft losing electrical power and the landing being aborted.