UPDATE:
Right, SO. Two of you jinxed me. Someone made a comment about ovens going off like old milk, and someone else said something about leaving old food/fat in there. Somehow I managed to combine the two by forgetting I'd stored a carton of UHT milk right at the bottom of the oven when there was nowhere else for it to go. So now not only are my spicy bean burgers ruined, so's my oven. REALLY not my day (.../week/month/year/lifetime).
How the fuck do I clean this and make it usable again? sob