The three that most spring to mind for me are as follows:
Once when I was first starting I went along with my supervisor to do an inspection of a home that turned out it was boarded up. We got out and went to look at it, and affixed to the boards were warnings from the city stating that there was hazardous mold growing inside the home and it was dangerous to enter the property. We proceeded to walk around the outside of the home where in the rear we found a tree that was growing into the house itself. In addition, there sounds coming from inside that sounded like there were animals in there. Fortunately this wasn't my order, and I believe my supervisor ended up declining the order after explaining what appeared to be going on there.
My second story involves something I'm sure others have seen before, but I'll retell it anyway. I went and inspected a home that was so full of 'stuff' that there were only path ways. These pathways had been present so long that the carpet and pad was worn all the way through. It was possible, after climbing over 'stuff', to enter the kitchen and bathroom (which were disgusting). It was not possible to enter the bedrooms as they were too full of 'stuff' to open the doors. This was bad, but not nearly as bad as the first story in my opinion.
Finally, just the other day this happened. I was doing an inspection out in Forest Ranch, CA. It's about 15 miles up highway 32 to from Chico, CA. First to get there, all the maps and direction sites have you going down this road that ends abruptly at a steel gate crossing the road. This of course is only about 2 miles down the road from the subject. So, to get there I end up having to drive for 30 minutes down dirt and gravel roads up and down the side of a mountain to get to the subject. The subject was located on 20 Acres and was foreclosed on in June it looks like. It looked like the owners had just got in their car and drove away. I know this isn't all that unusual with REO property, but there were some fairly expensive items left at this house, as well as sentimental items and decorations. It looked like the home could still be occupied. The whole exterior was littered with personal property too. Well I'm out inspecting the property, walking around the lot looking at things, and then I start hearing this sound of metal pipes clanging together. This continued for about 5 minutes or so, then it returned to silence. The longer I was there the more it felt like I was in some 'B' horror film and someone or something was going to jump out of the bushes at me at any second. I was completely isolated with no one nearby at all and it was incredibly creepy.