When I was younger (about 14 or 15) my Dad would hire me and a couple of my buddies to clean out rentals where the tenants had either been either evicted or just up and left in the middle of the night. Weirdest things that I found in those were: a home made long bow, a sex swing along with various flavored lubricants, drugs and associated paraphernalia, a room full (and I mean FULL) of old newspapers and magazines just thrown in and a dead parakeet in a freezer. All of that goes with the general lack of basic cleanliness. As a kid, I could not believe that people lived like that. The amount of trash, rotting food, dirt, dust and layer upon layer of grime left such a mark on me that I never thought I would get into real estate. Once I learned about the other side of the business, well, here I am!
I also once did a high end flip in a mountain town here in CO. Beautiful home overlooking a gold-medal fishing river. Picked it up out of foreclosure. The tenants (who were technically squatters) had left piles of dog poop and puddles of pee all over the house. Big dogs, too. The best part of that house was that they had one of the pop-up hunting blinds right next to the bed in the master bedroom. That's where they were growing their weed. Vented to the window and everything. Could literally lay in bed and reach into their "grow". They had absolutely trashed the place but it made for a great flip (once we remediated the meth issues).
Saddest one was a quasi-hoarder house. Home was packed full of everything you could imagine. I think I went through 3 or 4 40yd dumpsters throwing out junk, old furniture, carpet, etc. Where it gets sad is the previous owner had abandoned family photo albums, old birthday/xmas cards, etc. If you flipped through the pages of the photo albums, you could see how the family began falling apart and the inevitable collapse of a marriage. I contacted the previous owner and he eventually came to pick up some old tools and such but left the photo albums and other personal/sentimental items. After talking with him, it seemed like he just wanted to close that chapter of his life and move on.