A weird floor dip in a 1950s house turned a 4 hour job into a 12 hour nightmare

I was putting carpet in a living room last week, a normal rectangle. The floor felt okay when I walked on it. But when I laid the pad down and started stretching, I found a soft spot right in the middle, a dip about an inch deep and three feet wide. The homeowner had no idea it was there. I had to pull everything up, cut out a section of the subfloor, and sister in new joists to level it. What should have been a simple install took me from 8 AM to 8 PM. Has anyone else hit a hidden floor issue that blew up the whole day? What's your fix?