I just stumbled on a case which can cause massive headaches and problems. In some cases which I have not yet pinned down, it is possible that instances are still left running even after all jobs are seemingly finished. Whether it is caused by a dangling spot request, freezing render in an instance or something else, I haven’t discovered yet. But what I did discover was an instance that ran idle for 3 weeks and generated a 200 $ bill for me.