The April issue of Chess Life has an article titled "The Night I Beat Two Grandmasters." The writer has an 1100 USCF rating. It was a simultaneous exhibition, so I don't know if that counts (if I were the author, I would count it).
Also, I knew a player who beat Bobby Fischer in a simul in the eary sixties.
Every now and then I read about weekend tourneys upsets where GMs draw or lose against much lower rated players.
Does not happen very often, but it happens.
Here's an example where a young student and National Master wins against a GM (2400+) in a miniature :
Source :
https://www.uschess.org/content/view/9624/544/
eric is my teacher!!