I do seem to run into these problems with acquiring quest items and their relationship with my inventory. Is it a Mac thing?
This time it's a beneficial bug but still went wrong! Got to the part in A Study In Blood where I had to retrieve Sabina's portrait, only to find (while looking for something else) that it had already got into my inventory! So I gave it to her OK and the quest advanced. I then enter the house to kill the artist, but he disappears up to bed before I can even start.
I exit the house and reenter, attacking him before he can get up the stairs, only to find myself mysteriously whisked outside his front door and being attacked by his unattackable guards. After this happened three or four times, I gave up! Obviously, the quest is out of sync and going upstairs is what he was supposed to do if I had tried to steal the painting as I should have done.
Both this and the previous problem I had with the guy outside the goblin burrow could be solved with an abandon quest button, allowing the whole quest to restart. Can I make a plea for one to be added when all the bugs have supposedly been reported?