Never witnessed one. Almost did: was at a girls' night at the home of a friend of a friend. Her toddler wouldn't go to sleep, and she dragged the kid upstairs, saying "you're going to get the spanking of your life." I left. That was purely and simply upsetting. I'm open to the idea that corporal punishment needn't be intrinsically abusive, but causing physical pain to a 3 year old for not going to sleep? That's abusive.
I still wonder if I should have said something.