Spent 3 hours chasing a hollow center in my chocolate cake before I checked the oven thermometer
I baked it 4 times and every one sank in the middle, so I blamed my mixing and my flour. Turned out my oven was running 40 degrees too low, and a $9 thermometer from the hardware store fixed it in one bake. Now I want to know if anyone else has a crusty old oven they had to learn the hard way.