Found in Version:
In the final step of this quest, you are given a choice about siding with Bartolomaeus, or refusing. Upon refusal, you slay him and plunge the Ancient Dunmer Blade quest item into him, releasing The Fiend. The Fiend gives you as a reward the blade you just plunged into Bartolomaeus' body, enchanted with his trapped soul.
The problem is that the actual quest reward given is a spear, not a long blade as the Ancient Dunmer Blade had been.
This contradicts 1) the prior quest item, 2) the spoken dialogue of The Fiend and Bartolomaeus, 3) the journal entries for the quest.
Interestingly, the item is fully textured as a spear as well--presumably there was some decision midway through quest creation to change the item type that was not fully communicated to all involved parties.
Not game-breaking, but strikingly inconsistent with everything else in the quest up to that point.
I run a couple of mods, although not many. The only one that I think could even possibly conflict with this would be Cast On Use B-Gone, but the version I have is only for the base game and does not affect Tamriel Rebuilt items, so it seems unlikely.