Board Thread:Lore Discussion/@comment-104.185.108.226-20150828184056/@comment-6881629-20150829090322

Picking up quest items early sometimes stops the quest from recognizing them, according to The Pursuit page having the plans can make Karliah very confused (there are ways to possibly fix it on the bugs list). The page for Mercer's Plans also says you can just put them back (but i would of though it would be quest item and undroppable).