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.
(Last week, almost this exact scenario happened to one of my customers.)
Apps like eBook readers, podcast clients, offline mapping programs and even Apple's own iBooks app (as long as Apple is following its own rules) would have its cached data wiped clean if ever the device runs low on space.
Hit the link below for Arment's detailed explanation of the problem.
Source: Marco Arment