PC Using the console enter: SetStage TGlarceny 80 to activate the quest to return the Left Eye to Delvin, then add the Eye to the inventory with the console (player.additem 1994f 1). Then return to Delvin Mallory to turn in the quest BEFORE killing Mercer. If Mercer is killed before turning the quest in, the quest may disappear from the journal as soon as his body is looted.
360 One can escape Irkngthand without taking the Eye, take the rest of your items back to Delvin, sell them, and go back to get the Eye off of Mercer's body.
In the Slave Pens, there are numerous Falmer huts that are very difficult to get into but impossible to get out of due to the collision box of player characters without the use of console commands. ( PC )
PS3 If The Elder Scrolls V: Dragonborn is installed, a potion of waterwalking or an item with a waterwalking enchantment is used when the Sanctuary is being flooded, the player will go through the roof. The way to fix it is to go through the right side of the exit tunnel.