usually if i accidentally destroy a hybrid that i have no spares of. even then i sometimes just leave it, some progress isn't worth losing.
also sometimes after the game saves automatically (e.g. returning from a visit to another town) i just shut off my ds because i don't see the point in saving again, i don't mind seeing resetti too much. i also do that if i start up my town only to change my mind about wanting to play.
In an old save file, I logged in as my mayor to my very second day in the town only to find a villager had moved in right on top of my one and only perfect fruit tree.
I'll reset if I accidentally kill a hybrid but I also will if a villager pings me to move out and they change their mind.. like no, you are moving. Haha