If they are going to hurt themselves, someone should say SOMETHING. Perhaps a word to the instructor after class, not pointing out any specific individual, phrased in a way that shows you are concerned for safety? Is that why it bothers you?

I often see people doing different exercises inefficiently and I find myself saying (to myself) "She'd be so much more efficient and stronger if....". BUT, I figure if they are out there doing it (rather than home on the sofa) and they are not hurting themselves (or others), then good for them! and I just need to accept it and move on (and keep my mouth shut). I'm sure that some folks wouldn't mind a few pointers, but unless they ask, I'm not saying anything. And what might work for me may not work for them. I'm sure I am very inefficient in the latest sport that I have taken up (skate skiing), and that all those old guys smokin' me want to tell me to bend my ankles more or glide less or whatever!