I've noticed that once you have completed certain quests or choose specific options in quests, the related items stay in your inventory flagged as quest items and cannot be dropped, sold, stored or otherwise removed from the inventory- even after the quest is completed.
Sandraven Holy Axe, Acquired in “The Cradle of Fire” from refusing to give the axe to the orc chieftain, failing the quest objective “Return the Holy Axe”. This one is particularly annoying because of its high value and heavy weight.
Dwarven Hammer, Acquired in “Wartime Treasure” from looting the dwarven ruin in the Lava Forrest. When you turn the quest in at the quest board in Caer Cyflen, you complete the quest, get the rewards, experience, and reputation- but the hammer remains in your inventory. While not super valuable, it is fairly heavy.
Encrypted Letter and List of Prospects, Acquired in “The Missing Piece” when looting the desk in Halman Summer’s murder room in his house. When you return to Lord Carran, he “takes” the documents but they remain in your inventory and cannot be removed.
Golden Key, Honestly have no idea where this was used, but it’s stuck in my inventory.
Oddly enough, the Temple Key from the end of “The Missing Piece” is flagged as a quest item but can be removed from the inventory.