According to http://www.uesp.net/wiki/Skyrim:Riften_Jail
In the rear of the cell there is a http://www.uesp.net/wiki/Skyrim:Shadowmarks indicating an escape route. To the left of the shadowmark is an activate button with the text "Activate Broken Shackle". If you use the button, the guards will be alerted and attack you. The button was likely intended to destroy the wall to give the player access to the tunnels beyond.
So basically this seems like a common problem. When will this be fixed?
