I have two teenage boys, so I know how kids are, but I had high standards for their behavior in public (even my autistic son).
From what I see on a daily basis, one of my pet peeves is when parents/adults think it's so cute to have "junior" push a shopping cart. Junior might not be able to see over the handle, but I suppose mom/dad thinks he/she is helping. So, junior's weaving all over the store aisle, blocking it, or even racing up and down the aisle and the parents are clueless. Those are stupid parents.
This thread also reminded me of why I stopped inviting a certain family over. They had three kids around my boys' ages (lower elementary at the time). The first thing their kids did upon entering my home was to jump on the sofa! This mom and dad's style of discipline was to plead with the kids - god forbid you hurt their little egos! It didn't help that all 3 kids were strong-willed.
The funniest - and most pathetic - display of this family's discipline (or lack thereof) was when we were at a farm. The daughter, who was about 5 at the time, went into the cow pasture. Cows are generally harmless, but they are big, so DD stands there, bothering the cows, and mom and dad are pleading with her to come out of the pasture. They didn't go in after her. She ignored them and they eventually got tired of pleading with her. The kids are teenagers now, and I wonder how they turned out. Not good, I'm guessing.