Save breaking bugs are bugs that either trigger two things: save corruption, where you can’t load the save anymore.
To be extra careful, be sure to save before entering Snowpoint City’s Gym, and do not save until after you’ve beaten Candice and left the Gym.
Japanese player Monaka explained on Twitter how they got stuck permanently in Snowpoint City’s Gym.
It can be triggered when solving Snowpoint City’s Gym’s ice sliding puzzle, just before challenging Suzuna (Candice), the cutest Gym Leader in the game.