User blog comment:EL TONYO/What is your overall opinion of Skyrim?/@comment-1960986-20111206220928/@comment-4644719-20111206233135

it works beautifully. However, there is a small problem. Some of the infinite quests are uncompletable. For instance, the "Fetch me that book!" quest given by the librarian Urag gro-Shub at the college of Winterhold sent me to a dungeon I had already cleared. This particular dungeon required the death of one of the Dragon Priests in order for the last door (which hid the book) to open.

Since I had already killed that Dragon Priest, and they do not respawn, I was unable to complete that quest. Nor was I able to find a "I can't get you that book" in Urag's dialog options.

The most profitable infinite quests by far are the ones given by the Night Mother at the end of the Dark Brotherhood quest. So unless you're a morally superior RPGer, don't kill Astrid when she abducts you! Though, I guess that if you were to fence the items obtained in a "The Bedlam Job" quest for Delvin Mallory at the Thieves guild, you might be able to turn a higher profit.