The actual edge case might be when discovering a new producer and *adding* a new connection... say `max_in_flight == 5` and `# producers == 5` and then you poll lookupd and discover a 6th connection.  If you don't receive any message on your pre-existing connections (which *would* call `balance()`) then l don't believe `balance()` will ever be called.

We often referred to this as "toggling" in and out of "low RDY".