A floor joist rotted clean through
A long-running plumbing leak under the hall bathroom had soaked the framing for years. The main joist directly below the tub had rotted all the way through and was no longer supporting the floor — and the surrounding subfloor was water-damaged as well.
This is active structural failure beneath a finished, retiled bathroom. The floor that looked solid on a walk-through was being held up by little more than its finishes. Left alone, it gets worse and more expensive — and it's not something a buyer can see.
