Your used game/rental game existing save stories.
By way of example. End of the 16 bit era. I had purchased a copy of a Lotus III driving game for the megadrive. Today it is not so well remembered other than being an earlier example of procedural generation, and a few select fans of game music (skip to track 3/6:29 for what many consider the best). In my case the high score table told me in a few letters each time how to access the hidden shmup minigame, something in the pre internet era I might never have discovered. Presented something of a dilemma when I eventually got good enough to start troubling the high scores as well.
More recently. A "too good to be true" deal on an xbox 360 slim saw me have to clean out smoker house dust from everything including the power supply, however that did mean a nice working xbox 360 as a result. It included a profile with a bunch of saves on it, one that I am actually still using (who needs online games). Exploring someone else's game of Skate 3 was an interesting affair, though if someone else has played with someone else's pokemon I am sure we are all interested in that one.
On a somewhat related note then back on the N64 then in Goldeneye you had to earn cheats by beating levels within a timeframe, some of them were fairly notable feats so a friend traded his cartridge with said cheats unlocked with another person in the school.
Maybe you encountered a copy of 3ds Resident Evil Mercenaries and all that went on there.
To that end you are invited to share some notable or otherwise interesting stories of your interactions with second hand games and the saves contained upon them.