After separation from the boosters that got the space craft into orbit, the main engine of the craft had to burn to head off to the moon. This engine could be maneuvered to “steer” the craft, but to propel the craft accurately (without yawing or pitching) the engine had to line up with the craft’s center of mass and the desired velocity vector.
The craft actually calculates the center of mass before the main burn by starting the engine and wiggling the engine a bit then checking the resulting change in the crafts attitude (using gyroscopes, etc.)
The software was designed to do all of this automatically and testing showed that it worked.
However there was a problem. After separation from the booster stages the craft would be in zero gravity so the main engine’s fuel was a big sphere of liquid floating in the center of the fuel tank. The initial burn for the calculation of center of mass could falter if the fuel wasn’t at the base of the tank where the tank connected to the engine.
Before the main engine could be fired the maneuvering jets had to be fired just long enough to press the fuel to the aft side of the tank. The software did this too, but they discovered that in some cases the software didn’t do it long enough.
Fixing the bug was too risky so close to launch, but the software could wildly miscalculate the center of mass and the subsequent burn of the main engine could send the craft into a fatal rapid tumble.
The solution they came up with was to tape a note on the control panel next to the main engine switch telling the astronauts to manually switch on the maneuvering jets to accelerate the craft before ignition of the main engine.