it seems Juice doesn't check for enclosure changes (perhaps only date?) when it checks, this becomes a problem.
Example one: The BBC put an incorrectly edited podcast up. Some people downloaded it (like me). They realized their mistake and edited the RSS feed to refer to another file. But they didn't add a new entry. They just changed the URL in the RSS feed. And Juice never noticed this, so it assumed it had downloade the podcast and I had to manually download it...
Example two: In another feed. Someone had made a typo in the URL - so when juice tried to fetch it it just sat there not working. When they discovered that mistake, they changed the typo to point to the correct file, but again it wasn't a new entry, just an edit of the old one - and juice never picked that change up either. So again i had to download manually.