Hat tip to Duncan Young for this photo from Lavenham showing a building you'd never get away with as a model. He suggests subsidence as the cause of the wonkiness, but as the ground floor is still square, I'm inclined to the idea that the timber frame of the upper storey has twisted over time.
Whatever, caused this plaster must have been restored a few times as it's good deal less flexible than wood. Surely, it would have fallen away as the supports move.
Anyway, model this and people will look at you oddly.