I played Super Smash Bros. Melee to 99% completion, took me well over a year, got all the characters, all the stages, all the unlockables, high scores in everything that kept score and out of the 290+ trophies in the game I was only missing about 4, 2 of which were from save files of games I hadn't yet played...
And then my sister rented the Sims for GC, which needed over 100 blocks of space on a GC memory card, but mine was a 256 that was nearly full, so she decided to delete nearly everything from every game I had ever played JUST SO SHE COULD PLAY THE SIMS ON GAMECUBE!
I didn't care about a lot of that, most of them were games I had rented once and never played again, but the SSBM save... that stung.