While iTunes 11.3 brought iTunes Extras in HD to the Mac, it was also introduced some rather weird bugs when users attempted to listen to podcasts. In some cases, subscribed podcasts would stop updating with new episodes and iTunes would become unresponsive while browsing through episodes in a list.
iTunes 11.3.1 addresses a problem where subscribed podcasts may stop updating with new episodes and resolves an issue where iTunes may become unresponsive while browsing your podcasts episodes in a list.
To address these issues, Apple has now rolled out iTunes 11.3.1 as a software update for Mac and Windows operating systems. You should find the update listed under your software updates, so best to get that loaded up as soon as possible if you've suffered any of the symptoms noted.