Breaking savegames is actually surprisingly easy. All it takes is a typo when setting a flag in a scene. It's easily fixable, but all savegames, which has seen that scene have now failed to set that flag correctly and worst case they are all beyond repair. That is unless you "hack" the game as in using the console to set the flag. Ren'py's savegame system is good for a released game, but maybe not so much for a game, which is updated. Actually that's not really the fault of ren'py, it's more like a general issue with visual novel engines or games in general.
Given how much and how easy things can go wrong, I'm actually more surprised that it doesn't happen more often.