I've gotten hot by Gank Squads in every FromSoft game I've ever played, but the worst griefing I've ever suffered from was probably when my younger cousin broke the lock on my bedroom door, ransacked my room, and stole my PS2 memory card, and overwrote/deleted EVERYTHING that was on it. Kingdom Hearts, Tony Hawk, Dark Cloud, ATV Off-road Fury... I had only thirty games at that point in my life and he replaced ALL OF THEM with Jaws Unleashed save files, and my mom and aunt had the audacity to tell me I should have kept better track of my things.
It's been 20 years and that event is still carved into my brain like it just happened yesterday. I don't share my console or PC with ANYONE anymore.