I really think that these little Yorkies no matter what age and how long they have been going to their correct little potty areas we designate for them they still, once in a while are gonna mess up.
I recently purchases a beautiful rug for our dining room. Just this past week I noticed Schatzie doing her little potty squat on it.

I was like "Schatzie, this is NOT your potty pad, it might look like it to you, but it's Mommy's pad and you are not to rest your little bottom on it and pee-pee" And proceeded to take her to "her" pad.
So after hours spending shampooing my new rug I think she has been ok. But I wont say that too loud as she might just pee on it again if she hears me.