Has a highly-rated GM ever lost to a low-rated opponent who didn't go on to become a GM?
Also, currently has there ever been a GM who lost to low-rated opponents on a consistent basis?
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There was one famous reported game supposedly won against Mexican GM Carlos Torres by an amateur, E.Z. Adams, who did not go on to gain additional fame in the chess world
But this game is "suspect" because Adams was Torres' financial backer. So there is a suspicion that the game was "thrown" by Torres, or even made up.
There is a book called "Great Chess Upsets" by Reshevsky, but unfortunately most of the games are not upsets at all, but one famous player against another. Reshevsky himself, in his first major tournament, beat Janowski even though he was prepubescent. Prior to the tournament Janowski bitterly complained to the organizers that they were spoiling the gravity of the event by allowing children to play in it. After he won, Reshevsky reporting starting jumping up and down shouting, "I beat a grandmaster! I beat a grandmaster!".
Some of the more notable upsets:
In 1905 Frank Marshall, a World Championship candidate, lost to the unknown Dutch player Dirk Bleijkmanns at the Scheveningen Masters Tournament.
Joel Fridlizius beating both Alekhine and Spielmann at the Nordic Masters Tournament in 1912.
At the great St. Petersburg Tournament of 1914, the winner, Alexander Alekhine lost to German lesser light Bernhard Gregory.
In the 1923 Scheveningen Match Tournament, Spielmann lost to the little known Dutch master George Fontein.
In the 1924 Masters Tournament in Merano, Rubinstein lost to unknown player Gyula Patay.
Max Euwe lost to Vera Menchik at the 1930 Hastings Tournament, thereby becoming a member of the "Menchik Club".
The Cuban master Guillermo Estevez Morales beat Mikhail Tal after Tal blundered the exchange.
Croatian master Josip Rukavina beat Korchnoi at the 1973 Interzonal.
The unknown Ecuadorian player César Muñoz beat Bobby Fischer at the 1960 Leipzig Olympiad.
Bernardo Wexler beat Fischer at the 1960 Buenos Aires tournament.
An unknown player named Robert Burger beat Bobby Fischer in 12 moves in a simul after Fischer fell into a surprising trap of the queen.
Israeli Alexander Huzman beat Kasparov in the 2003 European Clubs Cup in 24 moves after Kasparov blundered two pawns because he missed a mate threat.
A trivial example would be in simuls, where you can find any of a million examples.
Another case where there's quite a few would be in blitz/bullet games. In fact, someone I know once beat Gawain Jones (an extremely strong player, currently #74 in the world) twice in bullet (game 1 and game 2). Admittedly, she's not exactly low rated, but there's surely cases as you described.
Odds are there's hundreds or tens of thousands of cases that fall under these two categories.
The chess.com article The Greatest Chess Upsets, Part 1 also lists a number of these upsets, although they are fairly highly rated players.
The Quora question Has a chess grandmaster ever lost to an amateur? gives a number of examples, although it's worth noting most of these had special circumstances, such as time odds or simul.
James Mortimer beat a number of great players of his time, despite being not very good. However, it's unclear if he'd count since there were no real ratings at the time.
TLDR: Yes, but most are under special circumstances such as time odds or simultaneous exhibitions.
One thing about GMs is consistency so losing in a tournament game to a significantly lower-rated player is extremely rare and the rarity can be roughly calculated based on rating difference. So a 2500 losing to 1500 might be 1/100 or less. Also, the way tournament pairings are done makes it pretty unlikely, even in an open event and impossible in a class event, for a GM to even play a 1500.
So it may literally be true that zero GMs (in modern times with modern ELO ratings) have ever lost to a 1500 in a normal tournament. There are firstly not that many GMs, secondly the 1500 would have to play in something like the US Open and win his first game against probably a low master and then of course beat the GM and THEN not become a GM. So maybe never happened.
On the other hand, Perry Youngworth was a promising junior (14 or 15) who did beat a GM in 1978 US Open. He was either a low master or a high expert at the time so not a huge upset especially since he was an up-and-coming junior whose ratings often rise very fast at that age but still a surprise. Nonetheless, he never became a GM or even an IM, I think. So depending upon whether you want to call an expert beating a GM an upset, there is that example but if experts beating GMs do count, I am sure there are a lot more.
Becoming a GM is not easy and used to be a lot harder -- there are many well-known and respected players (for their playing ability) who, due often to inability to get to tournaments of the necessary level of competition, never reached GM but "only" reached IM level. There are players who won the US championship without ever becoming GM (John Grefe and Stuart Rachels are listed as the only two).