If you ever did that in machinist class you would get a s**tty chitty. Never leave the key in the chuck.
He'll do that a couple of times but after retrieving it out of the wall or the ceiling or (god forbid) his forehead, it will be natural to take it out as soon as he is done using it. One place I worked at, if you left the key in the chuck and stepped away from the lathe, you received an automatic one day suspension.
Klunk: When you are finished machining bronze or brass, clean your lathe completely, these materials are very abrasive to your ways and wipers. Nice aquisition though. You'll have lots of fun.
Hints to keep you from getting tangled up in it. Even if you aren't a newbie, this advice is good for everyone.
If you use strips of sandpaper to finish with, hold the ends lightly between your fingers rather than wrapping the ends around them. Roll up your sleeves. Remove necklaces and rings. Always wear safety glasses. *
Never stand in front of the chuck while it is running. Don't use a file on the workpiece next to the chuck unless you feel confident that you can remove the tip from your throat or the tang out of your palm easily. Watch out for those stringers from the finishing cut, they are razor sharp - make yourself a little chip rake even out of something like a coat hanger, it is better than using your hands.
Advice from a lathe geek.
*I had a friend that died at work. A plastic pulley that weighed nothing struck him flat in the chest and stopped his heart. He was dead before he hit the floor.