Look at the Baku Olympiad.
A GM is not an easy prey, not matter the rating difference.
It's impossible for a human to play at 3000 strength, so the only way for a human to get beyond 2700 is to get really, really consistent.
Multiple players have had a tpr (tournament performance rating) over 3000.
He also drew two 2500s in the olympiad earlier this month.
He beat two other 2500s, so 3 out 4 vs 2500s in that tournament. Good enough for a 2700 performance, so he lost a few points over those games.
Seems like you all forgot a famous game.
Hou Yifan lost to a the finance minister from Latvia which was only 2243, Hou Yifan was at the 2600s.
Almost a 400 rating gap, just like in the 2500 GM vs Carlsen situation, everything is possible.
You can trick Carlsen by playing for a draw while he tries his best to avoid one until he makes a horrible move just in order to avoid a draw. Then you might have a little chance, if any. David Smerdon did this to Carlsen, except he was still trying to draw even once Carlsen made a horrible move.
Carlsen is human, not an invincible all conquering chess machine. Despite being far away from say the top 10 in the world, 2500 GM's are still fantastic players in their own right. If they get to play Carlsen it is probably the biggest game of their careers, against the world champion, world number 1 and some say the strongest player of all time. You can be sure that they have prepared as well as they can and will be desperate to perform at their absolute maximum as a win would be something to tell their grandchildren about. As NimzoPatzer and Binary say, the Olympiad showed that it is unrealistic to expect Carlsen to win every game against such players.
I read about people saying that GMs could wipe the floor with all 19th century players, and that certainly makes sense. I wonder, how strong is Carlsen compared to a normal 2500 rated GM? Do they even have a chance? I couldn't imagine a 843 beating a 1200...
Well, I could, but the problem is that it's even worse. 1200 is a fairly low rating. Some days a 1200 plays like an 800 some days and like a 1400 other days. The 843 is even worse, sometimes playing 1200 and sometimes playing 600. Yes the 1200 will come out on top in a match, but the game will have lots of blunders and lots of winning chance for both sides.
But Carlsen is rated 2857 because he almost always brings his A game to every single match. It's impossible for a human to play at 3000 strength, so the only way for a human to get beyond 2700 is to get really, really consistent. Play 2700+ strength again and again and again and again...
What is the lowest rated player carlsen has ever lost a game against? Does a normal 2500 even have a chance in a slow game? Can they even draw? I know GMs are very, very hard to beat and most people can't even be a GM no matter how hard or long they studied. But look at that number next to Carlsen's name...