I've been worrying about that ever since I put my condo up for sale. The upstairs neighbor's toilet flapper stuck the other night, and I could hear water running down the drain for a few hours. I texted her, but I imagine she was asleep. Luckily she has to "go" frequently, and eventually she flushed again with the flapper finding it's seat.
On the plus side our sewer pipe should be free of floaters now.
