You're obese or overweight.
I've done a lot of research and spoke to a family doctor and apparently eating below BMR is perfectly safe and won't slow the metabolism down so long as you have body fat to lose..
If you're a healthy weight and have a healthy BMI with low body fat, then it's not advised.
'It all depends on how much weight you're carrying around.. The more weight, the more extreme of a deficit that can be run. For example, lets say you weigh 200lbs and have a body fat percentage of 25%, that would mean you have 50lbs of excess fat on the body and only 150lbs would be lean body mass. You burn on estimate 33 calories for every pound of body fat you carry, so that person could safely create a deficit of 1500+ calories per day. Now lets say you weigh 150lbs with a body fat percentage of 15%, this would mean that this particular person could only create a deficit of 700 calories a day otherwise they'd start attacking lean body mass and damaging metabolism.'
So lets take me for example.. I'm 208lbs, and have a body fat percentage of 41% according to my scales. This means that my lean body mass is 128lbs and I'm carrying 80lbs of fat around. So I could create a deficit of 2640 calories per day before attacking my muscle & metabolism!
Obviously my TDEE is 2500 so it's impossible to create a deficit that large, and stupid too! But if I wanted a 1500 deficit or even wanted to be silly and starve myself and have a 2500 deficit then it'd be safe for my body because I'm carrying so much body fat.
Hope this helps some people.