I know a 12-th level commoner. She's quite old. So old that she has 12 hit points... And she's CR 11.
What's her role ? She's the town elder. Actually, everyone in the village call her "grandma", and for a good proportion (around 1/4), it's true.
Why could she be CR 11 ? Well, she's old and experienced. Sure, she has strange combat-prowess class wise, but this don't matter. She's too old and weak for fight. With her Str, Dex and Con modifier, combat is not an option.
However, there's something to remember about experienced NPC. They have equipment worth a certain quantity of money. She has some pretty nice wondrous items -- and not
scythe of harvesting +2 or silliness like that, no wizard is going to make such items. No, she has rather things like
bag of trick,
medallion of thought detection, and some other items.
The other thing to remember is that she has lots of friends.
However, her CR don't matter. PCs are not supposed to confront her.