1. Check multiple RSS feeds at once ("refresh recursively") and have one or more of them fail due to some error (e.g. HTTP 403)
Visual indication that an error has occurred for specific feed, e.g. an overlay red exclamation mark on top of the feed folder with a on-hover tool-tip showing the error or
There is no visual indication. The error(s) can be seen only in Network log (which one doesn't normally look at).