Itr filed without quoting pan

on a lighter mood, must be an achievement to file ITR without PAN! that apart, there is little that can be given as an reason for such a mistake other than simply accepting the mistake.  You may reply stating your correct PAN and request for condonation of the error and waiver of penalty on account of no malafide intention on your part. 

You can also represent that this is ur first such mistake and department may therefore look at this liberally.