When I first got Pokemon Ruby I had no idea what I was doing. Made my starter my HM slave, sold my Exp. Share and used my Master Ball on a Graveler. But the most facepalm-worthy thing I've ever done was release the Groudon I just caught because I didn't want it at the time. I thought I could encounter it again in the wild smh.
Oh and on my save file that had 1000+ hours on it, well, funny story, it was the same Pokemon Ruby file. I played the same game for 3 years and miraculously didn't get too bored. I let my uncle play the game and he showed me something I thought was new. What I saw on the screen was the character inside the moving truck with boxes. Of course, after playing the game for so long, I completely forgot how it started. So I got all excited and saved the game. When I went to check my Pokemon, there weren't any. Then I realized. Oops. And to think, I was only missing Latias and Kingdra :/