My iTunes library had 1 extra song than my iPod. When I searched the problem I found out that one of the songs was duplicated (same album, same track number, same size) but the name was misspelled (one character was missing). The bad news is that I searched and compared each album manually to find that song.
It was a Celine Dion's song and the two songs were named Celine Dion... and Cline Dion...
When you buy albums from itunes online, you get interactive booklets along with it. these are not added to your ipod, but are counted in your number of songs, also, count how many items are checked in your itunes.