I was having this issue on my iPad running iOS 6. The available space kept filling up after I freed space. Turns out it’s a glitch with the iCloud backup system. If you manually start an iCloud backup and let it finish, it frees up a bunch of cache. I went from having 0 bytes free to 9.3GB free after running an iCloud backup manually
The new iPod’s play podcasts continuously, so if you decide to go to sleep with an hour-long podcast going and fall asleep, you’ll wake up with a whole bunch more podcasts marked as played that you never actually listened to.
Go to the main menu, then change the Shuffle setting from “Off” to something else. I picked “Albums”. This stopped my podcasts from playing over and over again.