Apollo 12 was struck by lightning 36 seconds after lift off, effectively shutting down pretty much all electrical power and leaving the rocket out of control.
Nobody knew what had happened and, even less so, what to do other than undertake a most likely fatal abort. John Aaron in Mission Control analysed the problem and uttered the now legendary instruction
"Try SCE to AUX.".
The response from the crew was ‘what the hell is that?.
After being instructed again, they flicked the SCE switch to Aux and all was well.
One of the greatest examples of someone keeping a cool head while under immense pressure.
history.nasa.gov