I got the "dark brotherhood forever" quest quite recently, and was told to kill a noble at Dragonsreach. Then I got the quest objective to approach the Night Mother again, and once again was told to kill a noble at Dragonsreach, but it was another contact this time. The noble, however, was the same guy I had just killed.
I thought "different people want me to kill someone who can resurrect himself! This might be something epic." Then I got back to the Night Mother, as I was ordered, and she told me to see the first contact again (the priest in Whiterun). That made me suspect something. I wondered if it could be some kind of bug, so I searched this forum for "dark brotherhood forever". And not until then I realized what it was: A random kill quest that repeats itself, for those who RP an assassin