What are some of your previous owner nightmares you have inherited?
In a house we rented a while back, the previous tenants tried (and failed) to install a dishwasher.  Apparently, they were too cheap to find an adapter to hook up the waste flow from the dishwasher, so they cut a hole in the waste line of the sink on the wrong side of the trap and epoxied a piece of PVC there.  Then they hooked up the waste hose from the dishwasher and let it sit on the floor.

This caused dirty dishwater to flow back into the dishwasher where it sat and stagnated, and if there wasn't enough stagnant water in the hose, sewer gases would leak up through the dishwasher.

They also failed at the wiring, so the metal parts would occasionally carry some voltage and shock you if you're just unlucky enough.  Good times.