The Five Finger Discount achievement is an achievement found in the Thieve's Guild DLC where you, "Successfully pick pockets at each of the most four popular shops in Abah's Landing." One of the shops (Leaping Frog Sundires), however, has only one NPC that you're able to pickpocket from and that NPC, for whatever reason, I couldn't pickpocket. Every time I tried to pickpocket her, it would say she was empty.
The first time I saw it, I thought maybe someone else recently came in and stole from her, so I killed her and waited a couple of minutes. No one else entered and she respanwed. I went to steal from her, but she was empty again. Weird, so I killed her again, somehow expecting a different result. She respawned and she was still empty. I killer her again, left the shop, waited for her to respawn while I was out of the shop, and then came back. Still empty.
After the third time of her being empty, I relogged and came back, only to find her still empty. Killed her, waited, still empty.
I was still able to steal from other NPCs right outside of the shop, so I know there isn't an issue of me being unable to steal. It's just this one NPC that I can't steal from and sadly, she's the only NPC to steal from in the Leaping Frog Sundires in order to get this achievement.