This is a violation of safety rule number two in my shop. If you can't see the picture then have a visit to the blog site.
Yes, it's true. If you leave the chuck key engaged in the chuck on the lathe (or any other machine) it's grounds for nuclear freak out session on my part. Think about what would happen if you started that spindle going at 1200 rpm. Yep, the chuck key would be flung through the shop and embedded in the wall or in your skull for that matter.
Don't let the key leave your hand unless you are putting it away. Go do a hundred push ups if you let go, even if it's just for a moment.