What they said: "don't leave the chuck key in the lathe"
My interpretation: "don't leave the chuck key in the lathe unattended"
What they meant: "if the key is touching the chuck and it isn't touching your hand, you're ****ed"
Lul. Sounds like a mistake that only happens once
Last summer when I was in China doing cnc-stuff..
One of the freshmen students visiting the shop did exactly that.. Left the key in the spindle, Proceed to press start cycle at 3500 rpm..
The chuck cracked the poly-glass on the shielding.. I was working at a machine about 20 feet away.. But it was so loud , BANG, I thought someone died... and I was afraid to look...

edit: no one died, but Tp4 almost died from a heart attack..