PC 360 PS3 The final item, and sometimes the Left Eye of the Falmer (final or not) can/will cause the larceny quests to bug. It is possible that the death of Mercer Frey is the cause of the glitch and there are workarounds for the PC and 360 .
Fixes
PC Using the console enter : SetStage TGlarceny 80 to give yourself the quest to return the Left Eye to Delvin, then add the Eye to your inventory with the console ( player.additem 1994f 1). Then return to Delvin Mallory to turn in the quest BEFORE killing Mercer. If you kill Mercer before turning the quest in, the quest simply disappears from the journal as soon as you loot the body.
360 You 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.