The smell of cat pee. That smell does not leave with its owner.
Weird round glass thingies in the circuit breaker. These fuses are older than the Kennewick Man. It means you'll probably need to replace ALL the wiring! That ain't cheap. (You want a circuit breaker full of black plastic switches.)
Cracked and/or crumbling foundation, or evidence of water leaks into the basement = Big Money.
Single-pane windows and/or lathe-and-plaster walls. The windows release huge amounts of heat, while lathe-and-plaster have no insulation. Either one equals a frozen fanny or huge heating bills in winter.
Vehicle parts (or the whole car) in the neighbor's back yard means a lower resale value in the future.