I turn the oven off. Then, I realize that I have warmed the bottom oven and put the food in the top.
I put the food in the bottom oven and turn it back on. I wait 30, then 40, then 50+ minutes and the temperature does not increase to the proper number. I am convinced at this point that my thermometer is broken.
Then, I realize that when I turned the oven back on, I turned on the top oven, but (of course) now my food is in the bottom oven.
At this very moment, my food is in the top oven and I'm pretty sure that's the one that's heated up. Maybe I'll eat sometime tonight.
We've had this oven for at least 5 years.
And the thermometer works just fine.