Mine was with an extreme hoarder. Clear mental health issues. Single woman. We're talking junk piled waist high, rusty tricycle, old toys, black bags (some decaying) full of crap, a stack of old yellowed newspapers in a corner. Large rooms so full of rubbish that the only path is a small foot-width (that's a foot width, not 12") at one side of the room to move through.
Other houses I've seen included having cat-spray all throughout the house + aerosol spray paint fumes sprayed inside the house (with all windows shut), and another with the constant smell of hard drug use.
What horrors have you seen?