Marco Arment, developer of Instapaper, explains why iOS 5 could mess up Instapaper and many other apps. If an app stores a lot of data in Documents, Apple now slaps its hand because Documents can be backed up by iCloud; but if an app stores a lot of data in Caches, iOS 5 can now delete that data at will. So any app that moves its data store from Documents to Caches can lose that data. Apple argues that this is okay because Caches is for data that can be reconstructed — say, by re-downloading it from the Internet. But what if the deletion happens while offline? Someone with a Wi-Fi-only device could store stuff just before leaving the house, only to find it gone later. And, as Marco points out, Apple deletes the data but the app developer receives the hate mail.