Are you asking specifically about experiences from the player side of the table, or are you counting DM screw ups, too?
Most of my gaming has been on the DM side, but I do have a few memorable stupid-me moments as a player. One of them involved me playing a GoO Warlock named Kaid. The idea for his patron, Vorshiathrax, was partially inspired by Hermaeus Mora from Skyrim's DLC, Dragonborn. Books sacrificed to Vor appeared in his realm, but he valued unique and original manuscripts much more than copies.
Our party snuck into the house of a seaside town's mayor one night. We found his journal and took it along with a handful of shiny trinkets (...we weren't exactly the good guys in this game). Kaid hadn't done any sacrifices in awhile, and while the boring old journal of a provincial politician wasn't the most interesting thing Vor might read, there weren't any copies of it at least!
The next day, we had occasion to meet the mayor in the open. He lamented the loss of the journal, and after some prying he confided it contained code language used to unlock the main vault of a powerful mercantile guild he used to work for.
Kaid earned some very upset looks from his companions. Fortunately, another adventure was just on the horizon!