A great tip for all you thieves.
Here's a good way to level up your pickpocketing skills.
Get yourself arrested in Riften and break out of your cell, if your not a good lockpicker then it would a good idea to have the skeleton key. There will probably not be any guards on the floor your on, but if needed kill them if they spot, what are they going to do, arrest you?
Any way go over to the cell where Sibbi Black-Briar is being held and start to pickpocket him through the bars. Don't worry he won't be alerted if he catches you pickpocketing him neither will he be unable to pickpocket if he does, so just go ahead and take his stuff, and then give them back again, and take them again and etc etc. Then when you reached level 100 just go to our prison bed, take a nap and wake up a free man.
Congrats, you've found a game breaker.