Ok, for instance the transformer feeds 4 wire, 3 phase and N down a row of poles a mile long, at the end of that run there are 3 houses.
Each house uses a different phase and the same N.
All works well, N is bonded to 0v , normally the star point of the transformer windings. max of 240v from the star point to each phase conductor
If the N cable breaks along that mile run, the houses at the end of the run are still sharing the same N connection but now it is not tied to 0v
Depending on what loads are connected to each phase, the N voltage will drift towards the phase voltage of the phase with the lowest resistance load on it.
The effect is that the N conductor voltage floats seemingly at random from 0 to 400v relative to a phase voltage. (0 to 240v relative to earth)
It is more common to happen in rural locations where the supply cables are separate and 1 can be broken without affecting the other cables, but it is not unheard of in towns etc.
If you end up with a farmer on the phone saying can you have a look at my electrics, the lights keep going bright and then dim as if they are possessed, tell them to turn everything off immediately before things start to go bang!!