It's anecdotal of course but during a first body scan for a mindfulness course I got a panic attack. I stopped and on subsequent days did shorter meditation exercises before attempting another body scan (guided by audio, think it took like 35 mins), which was ok. It's not hard to imagine that if I would've tried to "push through" and hadn't stopped the body scan the first time as well as continued doing it on following days that this would have negatively affected my mental health.