[gumby]

By the way, it works that way by design. iOS will attempt to fill all the storage, and will throw stuff away (essentially on an LRU basis) as needed. Free storage is wasted storage, just like having a lot of physical RAM free is wasted in a virtual memory system.

This is why I was amused by the breathless articles last week about how to "free space on your iPhone" -- essentially by clearing the cache by golly!

(If the 16GB is filled up 100% and things fail, that's an actual bug though. You might really have filled it up with stuff that doesn't use iCloud, leaving too little for the iCloud apps to work with)

BTW all of the above is orthogonal to "optimize for space / use lower resolution on portable devices" that others have helpfully mentioned.