This oxygen tank was originally installed in the command module for Apollo 10, but they decided to swap it out for some upgrades. While taking it out, the support for it broke causing it hit the shelf and drop a few inches. They didn’t know it, but this damaged some of the plumbing on the tank, but not in a dangerous way.

Then when prepping to install that tank on the Apollo 13 command module, they ran a test to drain the tank, but because of the damage above, it only drained 90%. To solve this, instead of finding the damaged plumbing, they turned on the internal heater to just boil the rest of the oxygen off. But they were running the heater at higher voltage (65V) than the 28V on the command module. This caused the thermostat designed to limit the tank temp to 80 deg F to fail. The heater was on all night and estimates are that the tank got up to 500 deg F.

This didn’t cause any visible or measurable damage, so they didn’t think much of it. But in reality the high temps had degraded the insulation on wiring to the fans inside the tank used to mix it. They were now vulnerable to short circuit. The already compromised tank was installed in the Apollo 13 command module.

When they mixed the tanks during the flight, the wires did in fact short out which caused the insulation to start burning. About a minute later the fire worked its way out of the tank which breached it, and that’s what caused the explosion that the astronauts felt.

And here is the kicker- the fans inside the tank and the procedure to mix the tank were there because they were worried that the heaters wouldn’t operate efficiently in zero gravity to keep the oxygen in a super-critical state. But by then they had gathered enough data to show that this problem was negligible, so the mixing procedure that caused the explosion wasn’t even necessary anyway. They did make upgrades to the oxygen tanks to address this issue, but they also stopped doing the tank mixing in subsequent Apollo missions.