I... Oh God, this is embarassing >_<
... I replaced my Mewtwo's Recover with Strength in Pokémon Yellow
I was very young >:l
Oh, I also killed Articuno and saved afterwards, instead of turning the game off. I did the same in Pokémon Silver, I attacked Suicune with my Dodrio's Tri Attack, I didn't think it would make a lot of damage since it was level 39, but I KO'd Suicune.... and saved afterwards... >_<
Luckily I restarted both games and didn't make those mistakes again.
And more recently... I killed a shiny Durant in Pokémon White... I regret that so much ._.