One of my most vivid early childhood memories is being at our local bank with my parents, and an old man who worked there told me nicely not to touch the delicate ornaments on a Christmas tree. I was mortified and cried hysterically. But my parents didn't get all pissed off at the bank guy, and tell him how dare he tell their child not to do something. The guy simply saw a kid about to break something delicate and corrected the situation. And guess what? It was fine, and the message was received.
Usually I'm pretty vigilant and I try not to let my kids get into situations where it's necessary for other adults to correct their behavior in the first place. But occasionally it happens, and it's very effective when it does. I feel lucky that my kids have other adults in their lives who can help raise them and teach them how to operate and interact in the world: parents of their friends; aunts and uncles; teachers; and even strangers. The less comfortable they are with the person, the more amenable they are to being corrected, and the more they seem to realize that they are not the center of the universe, which as I've said before is my highest parenting priority.
I feel like too many parents take ridiculous levels of umbrage to simple polite correction from other adults. I'm not talking about other adults screaming at and smacking other people's kids, obviously. I'm talking about another adult nicely telling your kid that they're being an asshole. And honestly, I think that's awesome.