IPhone playing songs not in any of the playlists

You could create a smart playlist that excludes files that are already included in another playlist.

So, in your case if you generally sync a playlist called "At Work" whcih contains what you normally sync, but then also throw a few others into the sync too, you could create a smart playlist that looks for any ticked track that is not already in the "At Work" playlist. Call this "Adhoc" or something, and add this to your sync list, and you should have a new playlist that shows anything that wasn;t part of your original sync list.

I just tried this on my iOS 5 iPhone, and I had no issue. I would try restoring your device at Settings -> General -> Reset -> Reset All Settings and see if that fixes some glitch or something with your iPod. But, this is a software issue, so returning your device for a new one shouldn't be the only solution.