The problem is not with apt but rather with the Packages file(s) on the g different Debian mirrors. In order to correct it (This is a workaround, by the time it reaches you it is possible that it will already have been corrected), edit the file(s) /var/state/apt/lists/*unstable*main*Packages, replacing the word optionnal (it appears two times in each file) with optional (remove the extra 'n').
HTH FWIW - Roie Marianer
