Top GMs defeated by amateurs?

I was wondering if there have been cases where some Top GM has ever lost to a much weaker player. I don't mean an amateur player with a rating of 2400, but amateur players with a rating of less than 1800. 

Some GMs regularly play on the internet and sometimes against very weak players (Carlsen has played against guys with 1300 rating).

Has it ever happened? And if not, what do you think would happen if let's say a 1500 by some sort of miracle defeats Carlsen on an online game? The 1500 guy would go viral? Would he become a sensation? Nothing would actually happen? What do you think?

Variance isn't real in the sense that when both players are doing their best it's either 100% or 0%. There's none of this uncertainty. Carsen will beat an 1800 rated player 100% of the time.

But humans are humans. They do things like play under the influence of drugs, or play when they don't care about the result and play ridiculous moves. Stuff like that.

So if a low rated player beat a top GM in an online game no one important would care. Sure maybe some non-players would care, but it wouldn't be viral or important, not to the chess playing community.

True. In an important live game a 1800 would never beat Carlsen. But in an online game it is obviously extremely unlikely but not impossible. Maybe they are playing a bullet game and suddenly Magnus's Internet connection fails. Obviously it would probably not be considered a real loss but I bet that the winner would be telling his grandchildren that he defeated one time the world champion LOL.

BTW it is kinda funny Carlsen's attitude when he plays on the Internet. If you know nothing about him and you just hear him talk you would assume that he is some random club player, because Carlsen often says stuff like "Mmm what am I supposed to play here? I don't really remember this variation" or "Yeah, my position is not good, I am probably losing this one", but obviously at the end he ends up winning 99% of the time.

Of course!

That's what every great game player awaits!  The honour of having shaken a prodigy's hand .

In a live game a 1800 could very well beat carlsen, and given enough opportunities, a 1800 will beat carlsen when both players are serious.

The reason this will never happen, is because Carlsen doesn't play against 1800 rated players, (besides a few show matches). But when given enough opportunities, eventually there will be an upset

In simultaneous exhibitions, 1800 players have a chance to score an upset, but it's a pretty small chance! I was a spectator at a 40 board simul given by Hikaru Nakamura a few years back and the score was 39.5 - 0.5.  And Hikaru barely paused to think!

Several years ago there was this game where GM Dlugy was beaten by a 1400.

As far as I know the 1400 was never confirmed to have been cheating and the opening trap he used was borrowed move for move from this game.

very nice game, thanks for sharing

Wow! What a game

The Budapest can be tricky!

Wow. That's super impressive.

I don’t think a 1800 has any chance of beating a Top GM, let alone Magnus Carlsen. 
Carlsen very very rarely loses, he even went 130 consecutive games without defeat, and that was against 2700 and 2800 rating guys. How a 1800 will beat Carlsen?