It's all very easy to say "I don't smack, ever" wait until you have a child who just likes to push all your buttons & NOTHING else works (and you've tried everything).
"Nothing" and "everything" are strong words, but never accurate in this scenario. No one can possibly have tried "everything", because no one knows everything. It would be more accurate to say that sometimes a child pushes all your buttons and none of your usual coping methods are working and you can't think of anything else at that particular moment. I have been there too and have smacked DD on a couple of occasions. IMO that doesn't make what I did right, and I have actively worked on developing more and better coping mechanisms so that next time the situation arises I don't need to resort to smacking, as I have other strategies up my sleeve. So I think that yes, sometimes in the heat of the moment parents do smack, or yell, or say things that they regret. We are all human and not bad people for snapping sometimes, but I also think when you do snap, it is up to you to work out why it happened and to prevent it from happening again if you justify smacking as a "last resort".