I didn't know that you could actually ignite brownies until I accidentally blew up a pan of them.
I had baked some brownies in a Pyrex dish, took them out of the oven, and set them on top of the (electric) stove top to cool. Later, while I was sitting in the living room, I heard a BOOM from the kitchen, and went to investigate.
Seems that the burner was turned on - low, but on. I don't remember fiddling with it, but my best guess is I bumped it or something, or it looked like it might be on, and in checking I turned it on? No clue. At any rate, the pan detonated and there was glass and brownie everywhere, and brownie in flames sitting right on the burner. Grabbed the fire extinguisher to put out the flames, and spent all that night (and a chunk of the next day) cleaning up the mess. Months later, when I moved, I was still finding extinguisher stuff in nooks and crannies - it made a far bigger mess than the glass and brownies!