I'm not sure if this counts, but my most facepalm worthy thing is having a stick my ass about using non-Unova Pokemon in Black 2. You see, in each generation I usually restrict myself to using Pokemon added in its region in order to keep things fresh, but in Black 2, due to the lack of decent Unova fliers I actually liked, I stubbornly ignored all fliers completely in order to keep my team "pure" of non-Unova Pokemon, which really screwed me over since there is crazy number of fighting Pokemon users in that game, and I always ended up struggling against them since I didn't have a Psychic type or flier.
This led to difficulties in my fight against Marshall at the Pokemon League, where the Scrafty who I thought would get me through the fight with the use of Zen Headbutt got wrecked, leading to a gruelling battle where I had to use a boatload of potions to win. Once I won, I shook my head and came to my senses, switched off the DS, deleted my save, started a new game, and caught a Zubat when I had the chance, leading to a much smoother run through the game. Crobat wrecked Marshall as expected, unlike a certain Hoodlum Lizard.
I've softened my stance on using non-region specific Pokemon a bit since then, so whilst I still mainly stick to my team being made up of Pokemon from a new region, I will use Pokemon from other regions if I have too, like in X when I used Venasaur as my grass type but the rest of the team was still from Kalos.