I actually have a scar as a result from a slining accident. ( and people think that this is safe for the user)
When I was first learning,I was still getting used to the proper release. One day, when I felt really confident, I gave a mighty heave on the sling, and forgot to release

Centrifical force kept the stone in the pouch and I hit myself across the shin, right below the knee. It hit hard enough to bleed.

I fell forward, got up and cursed in several languages.
to this day, I have an actual dent in that spot. I show it to anyone that is interested in my slings, just as a warning.