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We have 8 2 post lifts at our shop, yearly were grease all the vertical channels, and have a lift maintenance/ repair company come inspect all the cables/ switches.
For operation ALWAYS initially get the car off the ground and give it a shake/ rock, and make sure the lateral arm locks have dropped into place. Then once you raise the car ALWAYS set the raised car on the locks. If a cable fails or cylinder line fails it'll keep it from killing you.
The reason that car probably fell was when the cable failed and it dropped that couple inches it bounced and jostled it enough to fall off the arms.
In my 8 years at our shop I've always followed these procedures and have had 1 car shift in the air, itwas a 911when the rear arm shifted and shot out, it caught the car by the rocker which I had to have repaired/ painted (that was my fault as i didn't check the arm lock).
another time was the motor stuck going up with a cayenne, I caught that one before any damage happened by killing power to it. We have all our lifts setup with a way to quickly kill power just in case something like that happens.