What is the lowest rated player that has beaten a player rated 2800 or higher?
There are only 2 2800 rated players, Carlsen and Caruana. All you have to do is search for their results since they became rated over 2800. I'll bet one of the two has even lost to a 2600.
Here is an article in the spirit of what you were asking:
http://www.chess.com/article/view/the-greatest-chess-upsets-part-1
http://www.chess.com/article/view/the-greatest-chess-upsets-part-2
What is the lowest rated player that has beaten a player rated 2800 or higher?