Yesterday when I launched iTunes, it started downloading the entire HDTGM catalog to my drive again. My subscription settings had not changed, and this is the the only podcast that was having this problem. To prevent it from re-downloading EVERYTHING, I had to set my subscription settings to limit the number of episodes on my computer to "10 most recent."
I'm curious whether this was an RSS problem, because the same thing happened to the Planet Money podcast a couple months ago... suddenly everyone who subscribed to that podcast started downloading every episode in their archive, and they ended up actually having to yank most of their older shows to limit the downloads. They said it was an unexpected side-effect of some feed changes on their end.
(Or is it just a fiendish plan to pump up HGTGM's iTunes stats mu-ha-ha-ha?)
Anyway, iTunes sucks.