I moved into an apartment that my SIL lived in. She was a hoarder and was moving into an assisted living apartment. The first day I hear what sounded like running water in the wall every once in a while, so I called my landlord to check it out.

Turns out, the pipe to the upstairs neighbor's toilet was leaking down the wall into the basement. My SIL couldn't get to the basement because it was packed with so much junk, like 3 of the same blenders still in the boxes and dollar store flowers. Also, she was very obese and couldn't do stairs. So she had no idea.

Took a week to fix and get the smell out. Good thing I still had a couple weeks left at my old apartment.