Getting roped into one-sided card trades by people I didn't know very well. That happened a few times.
Dropping my Game Boy into a toilet 'cause for some reason I wanted to try to hold onto it while pissing instead of setting it on a counter.
Any moment of games crashing after not having saved in a long time, or falling asleep after a long game night without saving.
Also, I was aware of missingo at a really young age thanks to my brother, and would frequently abuse rare candy duplication so I never had to train pokemon other than my starter. This made grinding in later games kinda difficult for me. When I was 12-13, I went back to the first gen games and noticed the Hall of Fame glitch after a particularly long playthrough that I eventually decided to dupe in. I panicked and erased my game and was really sad about it lol
I wouldn't say I had any particularly bad experiences, though. Except for a tendency to lose the games, which really sucked. But for the most part I always managed to hang onto one version in each generation, so it wasn't that bad. This has haunted me to this day, as I lost my Red version somewhere 4-5 years back, although I'm fairly confident it was swiped in that case.
Meanwhile I still have some really ass games lying around from 20 years ago lmao