The iPhone is synced with another iTunes library error Message

Whenever I select to manually manage music and videos, I get an error message that says "The iPhone is synced with another iTunes library. Do you want to erase this iPhone and sync with this iTunes library?"

I haven't connected my iPhone to any other computer, the one I am using now is the original it was connected to (Windows PC), why is this message appearing? The only reason I can think of that it thinks it is a new iTunes library is that I updated to iTunes 11 recently. Is there anything I can do to fix this without erasing my iPhone?

The fact is you are switching to manual syncing, thus everything will be removed and synced back on your device. You don't have to worry if everything is in your iTunes library.

Or you could leave it to the automatic version of sync and you don't get a warning like this.