UPDATE: Nokia has released a firmware to address this problem.
There seems to be a major flaw in the WP8 OS in how it stores data. If you look under the phone's data storage, you'll notice one section labeled "Other". This section seems to randomly fill up over time, growing in size, and there's nothing much you can do about it. With my Lumia 920, there's 32GB of storage so this flaw doesn't hit me as hard as owners who have a WP8 smartphone with less storage. Still, my "Other" section has grown to 8GB so it's definitely a concern.