I’m coming to the conclusion that this type of error is caused by using multiple systems to access the same Discourse install. I regularly switch between my desktop Mac and my MacBookPro when accessing varions Discourse incarnations (up to 12 at the moment reflecting my professional and hobbyist interests where I see this error on all of them) and Unread/New counts are never accurate. Wondering whether the serviceworker.js script is at fault and not coded to cope with such user behaviours.