Jin:
You chew your hair and sometimes you tie a knot in it, that needs to be cut.
Suga:
I don't know it's a bad habbit or not , but you'll easily get depressed
Rap Monster :
When you're nervous your telling people things you don't really mean
J-hope:
You usually overwork yourself
(Sorry i don't find better picture)
Jimin:
When you are nervous you bite your lips until it starts to bleed
Taehyung :
You chew your nails
Jungkook:
When do you stress you smoke