Do you remember Pokemon Rumble Blast? It's a Pokemon spinoff that came out in 2011 for the 3DS. Many say it sucks but I always liked it. You play as toy-pokemon and if you use Streetpass, you can be in a toy-pokemon store as your Mii.
>play Rumble Blast a lot as a 10 year old
>lose the cartridge like the idiot kid I was
>be 22, kept the old (now decrepit) 3DS and have a new one I got in my late teens
>update my Mii in the old 3DS by giving her my current appearance (important for later)
>get nostalgic and decide to buy the game again on eBay
>boyfriend surprises me with the game because I brought it up to him
>fuck it, guess I now have two cartridges and we'll return the extra one whenever we have the time
>research how to finish Rumble Blast 100% because I enjoy completionism
>find out you need two 3DSes because mythical Pokemon only spawn after a set number of Streetpass encounters
>guess I'm in luck
>cartridge A with new savefile in my new 3DS for me to play, cartridge B in the old 3DS just to activate the Streetpass thing
>try the Streetpass feature, but something odd happens with cartridge B
>of course cartridge B still has the savefile from the person who sold it, which I didn't wipe since I play on cartridge A anyway
>however Streetpass is connected to the console's data
>so cartridge B dug into my old 3DS and used the data I had from my original Rumble Blast from when I was 10 (this doesn't alter the savefile as it only appears in the game's "Streetpass toy store" as opposed to the in-game world)
>this includes my Mii before I updated her
>in other words, when connecting the 3DSes with both games, my Mii in the new 3DS/cartridge A encountered my 10 year old self's Mii, with the edgy nickname I gave her back then and all
>get to see past me's Pokemon because apparently Rumble Blast's Streetpass datas include your Pokemon collection
>get to encounter my old Pokemon in cartridge A (can't collect them but doesn't matter)
I am never wiping that data, ever