I was debugging an invisibility-related bug today. When connecting, Gabble retrieves a list of privacy lists from the server, and if no suitable invisible list exists, tries to create one. Only once it has successfully found or created an invisible list does it register a handler for privacy list pushes.
I don't think this can cause any user-visible problems: there's no window between it determining a list is valid and registering the handler in which a push invalidating the list could be received.
But, if we create a privacy list during connection, we get a push for that newly-created list... before we've registered the handler. So Gabble sends back an IQ error, claiming it doesn't understand privacy lists... which seems wrong, and made me panic.