In addition to 3rd party app data, iCloud also stores data for a few Apple apps and services, and if this data gets corrupted you can wind up with a very unstable iPhone (or iPod or iPad). Unfortunately Apple doesn't provide a way to let you tell iCloud to reset this data. Short of permanently disabling "Documents & Data" syncing with iCloud, the only way to fix this is to delete the offending data from the [home]/Library/Mobile Documents folder on your Mac and reboot your iOS device.
The Full Story
A few weeks ago I began experiencing a recurring Crash on our iPod touch here. The issue was that the "SpringBoard" would crash. For those of you (i.e. most of us) who don't know what SpringBoard is, that's the app that always runs on your iOS devices and manages the "home" screen. All those icons, etc., are you interacting with a program called SpringBoard.
Think of SpringBoard similar to the Finder on your Mac — it's always running. When SpringBoard on iOS crashes you are briefly brought to a black screen with the Apple logo and then right back to your "home" screen. Jailbreakers are likely to be quite familiar with SpringBoard because there are many hacks to change its functionality, but on this one, to be clear, there was no jailbreaking involved. This was a by-the-book iPod touch, no mods.
This crash was happening regularly, sometimes as often as every 10 minutes. The Apple Store first recommended we wipe the device of all data and NOT restore from a backup. Problem still happened. Then the Apple Store replaced the device, and again we did not restore from a backup, and again, the crashes quickly resumed.
No restore from backup. Brand new hardware. Same crash. Diagnosing this just got interesting.
Thankfully, we had allowed iOS to "Automatically Send" Diagnostics and Usage data to Apple. By allowing this, we also get to see some of the history of this data, including crash logs. Looking at the series of SpringBoard crash logs we saw the crashed thread referencing "TextInput" quite a bit. Digging, then, into the iCloud synced data folder on the Mac (located in [home] / Library / Mobile Documents) we found a com~apple~TextInput folder. Eureka!
Matching the iOS crash log with the related file on the Mac we were able to fix this
We disabled Documents & Data syncing from the iPod temporarily, deleted the com~apple~TextInput folder from the Mac, gave it a minute to sync up to the cloud, and then rebooted the iPod. After re-enabling Documents & Data syncing, all was well.
The Lesson
As we've seen with Mail preferences on the Mac, Documents & Data syncing can wreak havoc on iOS if the synced preferences and data wind up getting corrupted for some reason. Thankfully deleting these from the iCloud (i.e. Mobile Documents) folder on your Mac will help to resolve this problem.
Bugs-in-the-cloud image made by Bryan Chaffin with help from Shutterstock.