I have encountered a bug after doing the "Loot the Mages Guild" quest for Thieves Guild in Ald'ruhn. I had entered the guild under 100% Chameleon and stolen the tanto without any problems - the guarding mage turned hostile but I had no reason to kill him (especially that it was after the duel with Trebonius, so I preferred to avoid killing "my" mage). After I entered the guild next time (also under Chameleon, as I was doing the other Thieves Guild quests), the mage was still present and attacked the other mages, killing one of them. I had to reload and help to kill him. He should disappear after the quest is complete.