While Apple usually gets everything right, they occasionally drop the ball. Here is an instance of them dropping the ball: Marco Arment writes:

Instapaper has stored its downloaded articles in Caches for years, since I didn’t want to slow down iTunes syncing for my customers or enlarge their backups unnecessarily, and full restores don’t happen often enough for it to be a problem for most people. This new policy now locks me into using Caches: I no longer have a choice. But in iOS 5, there’s an important change: Caches and tmp — the only two directories that aren’t backed up — are “cleaned” out when the device is low on space. … A common scenario: an Instapaper customer is stocking up an iPad for a long flight. She syncs a bunch of movies and podcasts, downloads some magazines, and buys a few new games, leaving very little free space. Right before boarding, she remembers to download the newest issue of The Economist. (I think highly of my customers.) This causes free space to fall below the threshold that triggers the cleaner, which — in the background, unbeknownst to her — deletes everything that was saved in Instapaper. Later in the flight, with no internet connectivity, she goes to launch Instapaper and finds it completely empty. … When customers save an article with Instapaper, get a book in iBooks, or download a podcast with Instacast, they expect it to be there next time they launch the app. Even though it’s technically redownloadable, customers see that as their data — they put it there, and it’s theirs to remove if and when they see fit. When the cleaner wipes it out, it appears that the app has failed and deleted their data. And customers won’t know that it’s an iOS 5 behavior — they’ll understandably blame the app developers. Even though it’s not our fault, it’s certainly going to become our problem. There needs to be a file storage location that behaves the way Caches did before iOS 5: it’s not backed up to iTunes or iCloud, it’s not synced, but it’s also never deleted unless the app is deleted. I posted large excerpts of Marco's article because I want to help get the word out on this issue. This is generally more than I would like to repost from someone else's site, but considering the circumstances, I would think Marco would be fine with it.