I am always curious about this, not sure if its appropriate question, but I imagine others wonder the same thing. I usually don't care when it is simple exercises like biceps, but sometimes I see people squatting, deadlifts or the jerk and clean in very dangerous ways. (I.e. going up on tippy toes using calfs instead of using quads for the jerk and clean. You can kill your lower back by doing this)
Should I tell them, or just ignore it and let them possibly hurt themselves badly? I am not a personal training, so I don't feel it is my place to tell them what to do. But I know horrible and dangerous form when I see it.