Umm,,, That's hard, since there are so many, but I think the main one is with how I structured my code.
My game is chess inspired (https://itch.io/jam/gmtk-2019/rate/459874, if you want to check it out. And if you do, don't forget to read the section after IMPORTANT in the itch.io page's description, and don't forget rating the game), and I decided to use class Tile, Player, and the main one. In player I didn't really have much, other then the variable "hisTurn" (which I could keep global as a boolean "white's turn" or something like that) and was useless, and the having only the Tile class makes it a mess of a code.