Which is one mistake you see people make when it comes to fitness?

I’m a huge stickler about form, or posture, in all my classes. This means slowing down your movement and not just powering through. If you train in a hunched over position that is bad for your back and neck, you’re going to get strong in that hunched over position. People need to put more energy into making sure they have good form with a neutral spine and a strong core. Good form helps avoid injury.