This weekend, I had some friends over to play boardgames, and the cider with two cinnamon sticks happily sat on the back burner perfuming the air. I didn't feel like emptying the cider that night, so I repeated the trick Sunday evening.
The step I forgot on Sunday was to turn the heat off.
I woke up at 4 this morning, smelling something that smelled like carmelization carried a step further, and indeed, the moisture was gone from the pot, leaving a somewhat spongey disk. The cider had foamed up at the end of its life, making this disk about three inches thick. It reminded me a great deal of anti-cheesecake. Instead of a disk of dense white goodness, it's a disk of porous black evil.
Also, so much for making the house smell good. Whoops.
The good news is that nothing caught fire. Also, the pan in which I was heating the cider didn't get burned through. Also also, the disk of evil lifted out as one integral piece, making cleanup much easier than I feared.
This could have turned out a lot worse. Which might have more painfully taught me the lesson to remember to turn off the burners before going to bed. Hopefully the lesson sticks, anyway.