Situation: your class is sitting in the hall doing a pretty good job being quiet. Another class is completely out of control and their substitute teacher is screaming nonsense and no one is listening. The noise is deafening. Do you reprimand the loud class or ignore it because they're not yours?
I flipped on a class today... did the look-of-death-finger-pointing-your-teacher-would-be-so-disappointed-with-your-behavior. I began my speaking in a firm voice but was being completely ignored so I yelled. Really loud. They froze. They became quiet, but that's not really my preferred mode of dealing with loud kids. I immediately felt guilty for a) yelling, but b) also butting into a class and a teacher that's not mine. But then I didn't feel bad because they were so completely out of control. I decided that I would want another teacher to do the same to my kids if the situation ever presented itself.