Super Audio CD.
A CD with two layers. One layer contains the audio in the conventional way (Audio CD), the other layer contains audio in Mpg2 (similar to DVD).
Normal CD/DVD-ROM drives should only see one layer and detect the CD as a 'normal' Audio CD.
That seems to work out great except (in my experience) PIONEER DVD-ROM drives have problems.
They mistakenly mount the CD as DVD and none of the data is accessible.