Some of the common problems are dirty swimming pools.
One of my clients drained an icky pool like this, and found a car under the water!
Quite a few properties have kitchen counters which appear to have met the business end of a sledgehammer. Missing kitchen appliances and bathroom fixtures are common. One prior owner brought a garden hose inside and ruined the hardwood floors.
The worst story I think I've heard is one client who purchased a property in the desert near the end of the summer. After the purchase, my client sent a property manager to investigate whether it was occupied, etc, and discovered that the prior owner had apparently abandoned the property several months earlier, probably near the beginning of the summer. Before leaving, he had smeared yoghurt on all the walls and ceilings, then locked the doors and left. The mold was so bad, pervasive throughout the entire structure, my client had to pretty much demolish the entire house and build a new one.
I don't understand the mentality of people who do this sort of thing. Anyone have any ideas to enlighten me?