So I just thought of a couple things I did.
One was when I was really young, my parents had company over and they had a kid my age who they introduced to me. I decided to start a new game in Pokemon Red to play with him, but not save so I could keep my completed file. Yeah, you can see where this is going. When we get called off to eat supper, I decide to save the game so we can come back to it after Supper... it's not until after I got back from supper that I realized I had saved over my game. My very first Pokemon file. I don't remember all the details (this was well over 10 years ago) but I know I didn't react well.
The other thing is more recent. When I started playing Pokemon competitively again in Generation V, weather wars were in full swing. I had already had some success with a rain team and had started to hear about people using trapping Pokemon to gain the edge in weather wars. So I decide to try a sand team out with Excadrill to sweep. For my trapper I chose... Gengar. Yeah, I know. I decided to attempt a mean-look, hypnosis dream-eater set. It worked about as well as you'd expect, dropped to the lowest I've ever been on the ladder (well below 1000). Now I had made this team knowing it probably wouldn't work, but the sad part was I was still proud of my absolutely useless Gengar set. I've learned a bit since then, but holy crap I can't believe I ever thought that was a good idea.