So I fast travel to Riften to sell some loot and when I get there, I see a thief outside by the forge just standing there. I walk up to him and can instantly walk through him and he does nothing but stand there. Then out of nowhere, the entire town comes out and starts beating him and they aren't doing any damage, the thief does not die so for about a good 10 min I watch as all of the people from the Rift just beat the crap out of him. LOL

Obviously was a ghost and the town is ghostist

For the fighters guild random quest or any other form of radiant quest giver, if you are given a quest to kill the leader of an area you have recently cleared then you will have a marker on the dead body and the quest will be impossible to complete. This happened to me with embershard mine, valtheim towers, and other locations. If you arrive at the location and the person is dead, then you may have to reload a recent save in order to complete further quest.

aah....yes this happens more often then you would think. This also happens with radiant quest such as "Kill the dragon at location" or "Kill the giant located at this location" if you have killed the giant or dragon they will not respawn and will remain dead making it impossible to complete any further quest.