I've seen two distinct cases.
In the house I currently live in, the largest room in the basement has a floor that is uneven. Basically it dips lower towards the middle of the room. I presume this is caused by the foundation beneath settling.
When I went to view a room for rent, one of them was on the top most portion of a large old house. The room itself was small but after a minute I noticed it was slanted, yet the floor itself was even. It was quite an unpleasant sensation, visually the room looked to be normal but you get the sense of imbalance. The current tenant told me he had sheets wedged beneath one corner of the bed to compensate. What would've caused the floor to be like this? Tangent: in the same room there was a small fireplace. Isn't it odd to have a fireplace on the top level of a house and in a bed room?
Is there anyways a slanted floor can be fixed? In the second case the floor was made of wood.