One end of our den started sinking under the weight of a massive brick fireplace; the damage was repaired well before we bought the house and, as far as we can tell, nothing has changed in 13 years. I'm sure it's not square, though. I know that the upstairs floors--the downstairs are cement--all ripple and creak (the house is only 30 years old).

My uncle has a 130-ish-year-old house and NOTHING is straight. The kitchen floor at its worst had probably 10 inches of topography. He recently put in a new wood floor in the living room and arranged the planks in a herringbone pattern rather than a normal straight pattern so nobody could see at a glance how far out of alignment the walls are.