I'm just gonna update this to say that if a house is fed with 3 phase mains to the domestic outlets in a 230/400 system - i.e. some are on differing phases - extremely unlikely as the code advises against this - then, if someone turned on a simple 3kW heater, P-N on one phase, AND the neutral connection was lost to the supply Tx - then it is possible you could see nearly 400Vac across another outlet ( different phase ) with the current limited by the resistance of the 3kW heater.
If you happen to turn on another 3kW heater - you may not notice anything as it would be in balance with 200Vac across each.
If you plugged in an high impedance device - like a phone charger - it would see all the 400V and likely go pop.