Hi everyone,
The 'Correct Answer' as posted by LockerGnome is not the correct answer, at least when a PC is involved, as I have been able to reproduce the error afterwards. It only fixes the problem temporarily. How do you change its status?
This is how I was able to reproducet the "1 item could not be synced" message. I don't know if everything is a prerequisite but these are the conditions:
INGREDIENTS
One iPHone 3GS 32GB fully up-to-date, with iCloud, photo stream etc., all activatd.
One Windows 7 64bit PC with iTunes 10.5, all up to date, photo stream etx. active.
* The iPhone and PC must previously have been set to sync with each other, over-the-air syncing may or may not be a prerequisite.
* iTunes should be set to sync photos from the PC to the device. It may be a prerequisite that only certain subdirectories of the Pictures folder are set to be synced.
METHOD
1) Press the home and the unlock/on/off button on the iPHone together to make it take a screenshot.
* Taking a photo may also work but I haven't tired this yet.
2) Wait a few minutes for photo stream to receive the picture.
3) Launch iTunes on the PC.
4) Select the iPhone in iTunes.
5) Press sync.
6) Wait for the sync to complete.
RESULT
These things will happen:
* All the photos that are set to be synced from the computer will be uploaded to the iPhone, even if they haven't changed.
* At the end of the sync, the iPhone will display the message that items could not be synced, and to check iTunes for details.
Also, there is no additional icon iext to the iPHone that you can click on to find more information. Perhaps this icon only appears when syncing via wire.