So. You MIGHT overflow the drum if you overfilled the filling line, and had too many kernels. BUT: what would be much more likely would be rather than popping all the corn, they would overheat and get all funky, leaving you with a ton of unpopped kernels at the bottom: the popcorn needs room to expand into and once that stops, you're going to spoil the popcorn. The other element is important, because if there's a lid on top of the drum, nothing's overflowing. These things are designed to be able to pop some corn without having to watch it, while the concession person goes about the rest of their work.