Is it appropriate for an adult (stranger) to discipline a child if they see behavior that worries them?

I guess so.
I was waiting to pick up my son from school one day and I saw a kid about 11 years old beating up a smaller boy and as a father all I could think about was what if that was my son getting thrown around and I ran out of my vehicle and yelled at the older boy and told him to leave the other kid alone and get the hell out of here.
I looked at him like I was going to hit him and he just put his head down and walked away and the other kid got up and left.

I don't think it would have been ok if I did hit him and I'm sure that I would've been charged, but in that moment I wanted to help that little boy so I guess it worked out.

Yes, as long as the adult is not violent and is not abusive verbally. 

Hell no!!  If there is a violent situation, of course, try to stop it.  Otherwise, keep you own house clean.

Yes.  It takes a villiage to raise a child. This post was edited by Benedict Arnold at December 16, 2017 12:16 PM MST

I'd say probably at least 90% of the time that is a big fat, "NO!!"  (And I assume there is no physical contact involved or it goes up to 99.9999%

If a young child is climbing on the bags of flour at the grocery store (real situation), I don't mind telling them to stop doing that.

But my wife and I did go down and meet with the principle of the elementary school when a teacher said a few things to our youngest son that we did not appreciate.  

Our son never had another problem at the school.  (And the teacher wasn't there next year---which may have had nothing to do with us.).

Bottom line is that if you as an adult stranger are going to discipline my child, I am going to hold you responsible for that---so whatever you do had best be something that I agree with.

When I worked at our city's Mission years ago, I would tell children who were not behaving to cool it, IF and when I had already approached their mothers. Some mothers were too busy to notice their 'darling children' were trying to go up the stairs to where the residents of the facility stayed. I did that as a 'case worker" to prevent anything from possibly happening. If the kid/s were too unruly I would tell their mother to come back when she was able to find someone to watch their kids. 

This post was edited by SA (SuperA) at December 16, 2017 11:09 AM MST

I didn't have a problem with someone correcting my son if they were a teacher or a babysitter as long as they didn't put their hands on him.
I would do the same thing if there was no one else there to do it.