I had a fleet of stateful worker instances behind an Auto Scaling Group On-Demand floor for guaranteed baseline capacity, Spot instances above that for burst. Standard setup. Then we added a second scaling signal to fix a real gap in the first one, and the fleet started flipping size every 30–90 seconds. This is the story of why that happened, and why it wasn't a tuning problem. The setup Each instance in the fleet holds a fixed pool of "slots" a bounded number of concurre...