I stole it. Now I can't remove it from my inventory... (I think I can never remove it.)
This is a serious problem for me, I don't like being screwed out of 25 pounds of my inventory.

In my opinion, I think quest items could be sorted differently under your inventory, so nobody removes them by accident, but allow them to remove it regardless. What if it's a quest that has a really high skill check they'd like to meet, so they want to go gain 2 levels before they complete it? ect.
There are plenty of reasons that quest items should be able to be removed from the inventory for, and this would fix this problem.