Has a highly-rated GM ever lost to a low-rated opponent who didn't go on to become a GM? 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?
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.