I noticed a lot of people are having problems completing certain quests due to the fact that several radiant locations in the game can become bugged due to the "boss" (unique NPC) not respawning (after 10 or 30 in-game days) and/or not being replaced by a generic (non-unique NPC). I'm not sure if anyone has actually made a list of these radiant locations, but I thought maybe it would be a helpful reference and also useful in the case that the developers might someday be inclined to fix this problem. I know that several people have stated that it would be best if these locations were removed from the radiant quest system altogether. To the best of my knowledge, these are all of the locations that have this issue:

• Greenspring Hollow (Whiterun Hold): Sabre cat does not always respawn after the location is cleared (or the sabre cat is killed for any reason). This may be due to a bug where the respawn is completely broken if the player re-visits this location before the 10 (or 30) day respawn period passes. This was supposed to be fixed by patch 1.6, but as many people have stated there are still broken animal wilderness respawns.
  
• Knifepoint Ridge (Falkreath Hold): Bandit leader no longer respawns after Boethiah's Champion is killed during the quest "Bothiah's Calling", likely due to the fact that the champion becomes the bandit leader and doesn't respawn.
  
• Dead Crone Rock (The Reach Hold): A hagraven named Drascua is marked as the "boss" for this location and killing her before completing the quest "Pieces of the Past" will make this quest impossible to complete. In addition, after killing killing her during "Pieces of the Past", if the Companions send you to kill the leader here, there will be a quest arrow where the hagraven (boss) is supposed to be, which also makes this quest impossible to finish.
  
• High Gate Ruins (The Pale Hold): When Vokun (the dragon priest) is killed, any further attempts to enter the throne room are not possible (except by doing the tedious workaround), since the wall closes when the dungeon respawns.
  
• Uttering Hills Cave (Eastmarch Hold): Bandit leader no longer respawns after killing Linwe during the quest "Summerset Shadows", likely due to the fact that Linwe becomes the bandit leader and doesn't respawn.
  
• Rift Watchtower (The Rift Hold): The bandit leader at this location is an unnamed Orc. After he is killed once, he doesn't respawn making it impossible to complete a "Kill the Bandit Leader" radiant quest if you are assigned to this location again.
  
• Nilheim (The Rift Hold): The bandit leader at this location is usually a bandit NPC named "Guard". He doesn't seem to ever respawn if killed during the misc quest "Telrav's Request" or if sent here a second time to kill the bandit leader (often by recieving a bounty quest from Riften's Steward).