I have two as well.
They have mostly to do with sling design.
The first time I attempted traditional slinging I drove out into a near by field, making sure I was away from everything. I cut the cords from my staff sling in a moment of improvation, picked up a pebble the size of a small marble, and gave it a go.
Wham!!!
Suddenly my eyes were watery and I felt dizzy. The pebble had never left the pouch (the pouch design was faulty, made for trebuchets and large ammo), and struck me behind the head. I'm very glad that it was just a pebble. To this day I thank my lucky stars I didn't try slinging hard my first time.
My second story is similar to Tints. I was at an archery shooting match surrounded by compound bow shooters. I had spent most of the morning trying to make good with the officers of the range by volunteering so that they wouldn't mind me slinging. They didn't mind it seemed, so I went through the course slinging pine cones, until very suddenly a blister ripped open in mid-swing (poor retention cord design, and too much zeal), sending my pinecone barely humming over the heads of 20 or so people, and tearing the very sling from my hand. To my shame, I cried out in pain and everyone looked my way. The president of the range even came trotting over to see if I was ok. No one seemed to notice the rouge pinecone, but they all saw my soaring sling.
I hope you find this useful.
-Unsapien