One thing that often gets overlooked in urban or suburban areas, is that sewers will back up, especially if you have housing above you. City water and sewer rely on pumps to keep things moving, and even a well stocked hardsite can be rendered uninhabitable by a couple artesian toilets. The best way to deal with this is to jam some plastic tarp or other material into the outside cleanout trap, as the backup will affect the sink drains and washer as well. The good news is that it will take a long time to get there, otherwise storm outages would do this routinely, and we don't normally hear about this unless flooding is involved as well.