Lately I a curious fact has happened, is that every time I try to sync my iPhone via USB, iTunes generates unknown bugs me left and right.
Not sure if it iOs iTunes 10 or 6, but knowing that the same problem has been reviewed by sites like Unofficial reassures me.
I think the problem comes from inconsistencies when synchronizing applications you downloaded from your device, and generates incompatibilities with the iTunes library.
In my case each synchronization I had a different problem, and always at the time when the system was at the step where transferred iTunes purchases made with the iPhone, which had not been synchronized with my library.
As you can see in the screenshot, my last problem has been generated "Unknown Error (-54)" as the solution to the conflict has gone through change lel USB port which was synchronized in the past.
However, in my case the solution is simple, but if any of you still generates errors I recommend you to try the following possible solutions:
Connect your iOS device to another USB computer, as in my case port.
Restart the iPhone, iPad or iPod.
Restart the computer.
Use a different cable if possible.
All of the above.
If it still does not work you should uninstall and reinstall iTunes to the latest version.