You can also get this error on a correctly written CD or DVD as the burning process can add a few sectors to the end of the burn. That's OK as they are never read in while using the CD.
If you have verified the checksum of the ISO you have written you can start a terminal session with Alt-Ctl-T and enter:
ls -ltr *.iso
and determine the length of the iso in bytes. Then enter:
and then (subsituting the name of the iso and the name of your optical device):
cmp quantal-desktop-i386.iso /dev/sr0 -n $length
If there is no message the files are equal.
or as an alternative:
dd if=/dev/sr0 bs=512 count=$(($length/512)) | md5sum
and check the checksum against the Ubuntu web site as you did with the .iso file.
If the checksum of the downloaded .iso file matched the Ubuntu web site, and if Ubuntu produced a valid checksum as it was burning the CD or DVD, it isn't necessary to worry about any of this.