This was just something interesting that happened a few years ago. I had just installed a CDRW drive in someone's computer and they complained that a disc that they burned was having read troubles. I went to their desk and found a big greasy fingerprint on the record side. I took the disk over to a sink and smeared some dish soap on it and rinsed it off and noticed that the thumb print had also been transferred INTO the media. Apparently the laser could not penetrate the fingerprint so the outline became embedded in the media. It was pretty cool. I guess the user ignored the media errors when recording it.
So I did a full disc erase, came back an hour later and the thumbprint was gone and the disc sucessfully did a full burn with no errors reported.