Moved thread over to General Pokémon Gaming as it isn't so much about HG/SS, but more so all Pokémon games instead.
For me, I'm lucky to have a sister that knows not to touch my games, but her's instead. So, nope, can't say I've really experienced this one before. Though, it would be very frustrating to know you put in so much time and effort in one file and have it all gone in a simple blink of an eye because your sibling or friend started a new one. D: Although, with the newer games now, they have you press Up, Select, and B on the title screen before starting a whole new file. In HG/SS, they don't even warn you like in DPPt so even if someone did restart your game, if they didn't follow the Up, B, Select procedure, the new game file wouldn't save.