I have noticed a few times my trans shift into neutral while moving. Downshifting and upshifting, funny thing is I can hear and feel the shift into the next gear (i.e. the "clunk" into 1st) and then I hear the engine rev high and realize it dropped into neutral again.
I am really solid in my shifting and usually don't have this problem, so I don't know if it's me or the bike.