Do you concern troll at the gym? Would you ever stop someone doing an exercise/movement that was somehow injurious to them?

Last night there was a gymbro doing kettlebell squats with his heels up and knees twisting inwards on the descent. It was excruciating to watch. I didn't say anything, despite worrying about him injuring himself, I'm sure he would have told me to F off.

The only time anyone ever concern trolled me was a guy who told me I shouldnt be pushing my knees out over my toes during my squat *sigh*. I just smiled at him and said thanks.

This happen to anyone else?