ppl who bite nails and the skin around it: if you do it compulsively until your skin bleeds, then you may have dermatillomania/excoriation disorder/skin picking (whatever you want to call it). it’s an offshoot of OCD that makes you compulsively destroy your own skin. one of those things that isn’t widely known but isn’t an uncommon issue.
picking at nails/skin around nails is the tamest part of it for me, but still the most regular.