After clearing the rifle, I have the bad habit of grabbing the charging handle with a three-finger, palm down grip (like holding a tiny handle bar) and riding it home instead of slapping it down as you would when cambering a round. About half of the time, the recoil spring tension would pull my hand forward, and since I haven't let go of the charging handle, and since my fingers are riding along the fore end, the sling clip would act like a cheese grater and tear up the second knuckle on my left hand index finger.
EDIT: On at least one occasion, it happens when I charge the rifle because I grab the CH "palm down" as well.
tl;dr - operator error. You'd think I would have learned by now.