There are some side quests with different branches - and some of those CAN fail! o_o'
Indeed, as I've been discovering recently.

I've not hit level 50 yet, and I'm seriously thinking of restarting my game. I don't recall the quest I was on last night, but it asked me to meet someone. I get there, the arrow is present, but the person never shows. I even let the game cycle through 24 hours just in case there was a specific time.
I got frustrated, so I headed to the internet and it said if the character should die before you do the quest, the quest can never be completed (it was after the Markarth prison break-out).
Sure enough... character is dead. Lying right there on the steps from the blacksmith.
Now, I've quest log that'll never resolve, remaining open indefinitely.
I've also two items in my inventory I can never remove, given their owners are dead. One was the killer in Windhelm (still have their stupid charm) and the other was Dravin, an elf outside Riften, whose bow I discovered (on chance) but now can't return because he thought he was so cool as to fend off a blood dragon with a carrot. His body's been there since level 20ish or so.
Some quests were quite fun to do, and I wouldn't mind redoing them, some were downright frustrating, and repeating them dreads me.
Thus, I've been weighing the pros/cons all day regarding a restart.
I hope Beth offers a patch which prevents any quests from occuring if said character is dead, and allow us to remove inventory items as well.
Silver lining: Dravin's bow is "Flawless", the only item in my inventory to receive the upgrade.