Just hit a weird problem on my iPad: trying to install an app or upgrade existing apps resulted in a mysterious "Error 1004". Apple's official help is useless and the user forums are full of amusing cargo cultism ("reset your time zone!")
The solution seems to have been flushing Safari's cookies. Or maybe its cache or history. One way or the other, the iPad App Store had the wrong idea about my login. And now I have to log in to everything again.