I feel like this has always been a difficult thing for me. When someone does something they shouldn't do, I don't know how to honestly tell them without me looking like the evil mother hen, or something along those lines. It also makes it worse if they are young and/or possibly a bit oversensitive. Then, if they have their little army that will defend any of their friend's stupidity, it makes things worse, and well, I end up looking like D-bag when I'm not trying to be.
However, it's hard to ignore when they're doing something wrong either. I don't think it's good to smile, nod, and let it slide, because the person may honestly not know what