on the zx spectrum there was a football management game called "brian cloughs football fortunes"...me and my mate used to do a 2 player season and for formations instead of 4-4-2 ect,it would let us type 9-9-9....lol we rinced every game,both our teams were the top dogs...we were very proud to have done it ourselves.
another old classic was on WWF wrestlemania challenge(zx again) if you typed "hulk hogans yellow underpants" into the menu it would unlock hogan....honest guv.(its true)
then it was the console era with its game genie cartridges lol.....
i dont even play gta v without all the mods and stuff and invincibilty..(offline of course)....its what makes some games totally fun,i like to play properly until i get to the end,then i rince it for fun.