As of 2.5.1 RC2, setting the price of a shop item to 0 prevents Link from acquiring the item. In the previous build, 'free' shops could be used to give the player the same item over and over again.
I'm not positive what's causing the bug, but I'm fairly sure it has to do with the sentinel price for shops being changed from -1 to 0. Didn't it used to default to -1 for an item in the shop that isn't supposed to display?
