I lived in a place where someone managed to swap wires at one of the switchboards. There was no ground (not that uncommon), so the supposed 0 was wired to the ground at the sockets (that's what you should do). What it means with German/French-combo style socket is that phase (+/-230V AC) was wired to metal parts sticking out of a socket or any grounded appliance plugged in.

Everything plugged into affected sockets seemed to work just fine. Even a desktop computer + monitor. We only found out because roommate was getting electric shocks from the metal PC casing.