Bleak Falls Barrow: Dragonstone gone (main quest bug).
This actually appears to be two related bugs:
1) The mob that is supposed to drop the dragonstone in Bleak Falls Barrow may not be lootable after killing it.
2) If the Dragonstone is not retrieved from the mob at the time it is killed, the corpse may dissapear permanently, making it impossible to complete the Bleak Falls Barrow quest or proceed any further with the main quest.
I am not sure the exact trigger of either of these, but it may be caused by completing Bleak Falls Barrow (the area) during the Golden Claw Quest but before obtaining Bleak Falls Barrow (the quest). I, unfortunately, did not know that the mob was supposed to drop anything, and now my main quest seems to be irreparably broken
