16 Empty Bundles can be placed into a single Bundle, which will stack if the player's inventory is full upon emptying the Bundle

So, right-clicking items with the bundle selected will place said-items into said-bundle. 16 bundles can be placed into a single one, whether or not this is intentional, I don't know. However, if your inventory is full when you empty the bundle, the items will drop to the ground. Upon emptying this bundle full of 16 empty bundles, they dropped to the ground in a stack of 16.