Eh basic noob stuff:
I used a master ball on Abra
I don't know how but I got Pikachu angry with me towards the endgame and nothing would calm it down. Had to reset anyways because I swept through most of the game with Pikachu but I couldn't beat Giovanni and I didn't know when my other Pokemon would evolve.
I gambled all of my money away trying to get those prize Pokemon, can you see why I don't care about the Game Corner being gone and why I love the VS Seeker?
I knocked out the birds, I thought you were supposed to.
I spent months, possibly weeks, trying to figure out how to get past that grunt in Silph Co. so I could get to the thing that unlocks the door until I finally realized, "Wait, I just have to step on the teleporter, step on the teleporter again, and...walk...past...him..." Yeah, Gen 1 was the generation of mistakes.
I did better in Gen 2, I only named Silver ????, tried to evolve Togepi into Togekiss, hung around Goldenrod for a little too long, and, due to not understanding that some games have a postgame, I reset after beating Lance.
After that the only real mistake I'd make is not creating a team which XY kinda forces you to do with the gift mons, knocking out legendaries, and not EV train my Pokemon.