I've spent a lot of time looking at houses because I am constantly searching for THE house. You would not believe how many houses I've seen that say they've got X bedrooms, then two of those are a big open space in the basement with a super wobbly paneling wall built down the middle, with a metal garment rack or Sauder armoire in each room as a "closet." No doors, no legally required egress windows, sometimes not even four walls.
The absolute worst house I ever saw was a doozy. We were looking in a small town and working with a not-good agent who didn't feel like driving out that far. So she gave us the code and we went on our way. I didn't realize at the time that was illegal, but it seems obvious in hindsight. None of these houses were occupied, and it was only a few houses, but still.
We got to the last house and it was very run down looking from the outside. That's OK, we want a fixer. The code didn't work, so we called the agent who says it should work but isn't much help other than that. DH walked around the house looking for a back door or something while I waited on the front porch. He got in and let me in. I asked him how he got in and he pointed to a solid wall in the kitchen. I said "uh...how did that work?" Turns out there was a fire in the kitchen (you could see the damage very plainly except on that one wall) and apparently one of the windows had broken. Instead of replacing the window or even boarding it up correctly, they leaned a sheet of paneling up against the wall, put one nail in, and called it good. When DH saw the boarded-over window, he touched the paneling and it swung in completely. It was a huge window (old house), so he walked in and opened the door for me.
The house was absolutely trashed. It had to have been lived in trashed, too; this wasn't just getting back at the bank putting some holes in the drywall. In the living room, the carpet was brown and caked with dirt, hair and what I think was cat poop, except for where furniture had been. In those spots it was bright blue. There was a second floor, but there was no first floor landing to get up there. Well, there was a piece of paneling thrown on the floor. We took a big step over the landing to the first stair to get upstairs.
Upstairs the garbage was literally knee deep. We didn't stay long, but from a quick glance it was obvious that it was mostly pron. Of course the walls all through the house were covered in graffitti, trash was everywhere, etc. It was by far the worst house I've ever seen.
There was one other house that I still consider the one that got away. It had been converted into apartments and someone was still living in the upstairs apartment. It was pretty solidly wall to wall stuff, although not trash. Just stuff. It was great, but the basement had this weird room under the stairs filled with trash bags. Looking back, I wish I'd gone for it, room full of trash bags or no.
When my parents bought the house they currently live in, it was an absolute wreck. The couple who owned it were divorcing, and the deal was that the wife got to live there until it sold. She went out of her way to trash it--she had lots of dogs that she didn't let outside, trash everywhere, walls caked with who knows what. There was a pool; it was filled with dead mice and frogs. The yard was overgrown. There was stuff everywhere. The basement had water issues. They saw something in it, though, because they bought it and cleaned it up. Almost twenty years later, they still live there and just remodeled the kitchen. I think the reason it was able to be cleaned up was that everything was great quality, it was just filthy. I don't know if elbow grease would have worked as well if it had been builder grade finishes.
ETA: Remembered one more. There was one house we went to see with what sounded like a big dog inside. The agent knocked several times and it barked, so that was how we knew it was in there. She was afraid to go in, but we braved it. The dog was fine, but as we went upstairs to see the third bedroom (a loft with no doors), a head lifted up from the bed. A teenage girl was sleeping in there. DH was in front of me, so I hope she wasn't scared thinking someone had broken in. He said "Oh, sorry!" and practically shoved me down the stairs to get out of there.