Applications on iPhone or iPod touch may close unexpectedly because the device has begun to run low on application memory. This does not mean there is too much data stored on the device, but simply means that it is running too many memory-intensive tasks.
Some examples of this issue:

If you have many Safari web pages open on your iPhone or iPod touch, Safari may close and the iPhone or iPod touch will go back to the Home screen (where it shows all the application icons).
When listening to music or podcasts, watch video, etc. playback may stop and the device may return to the Home screen.
While using Safari or Maps and listening to music or podcasts, iPod audio playback may stop.
Applications purchased from the App Store quit immediately after you tap them.

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2. iPhone and iPod touch: Mail may unexpectedly quit if webmail account settings are synced
3. Syncing and arranging applications from the App Store
4. iPhone and iPod touch: Songs or other audio content are skipped during playback
5. Overview of syncing iPhone or iPod touch with your computer