Lots of films, music CDs and even computer software have hidden surprises on them which are called Easter Eggs.
A search in google for "Easter Eggs" will find web sites that list them all.
For computer software you often you need to press a certain set of keys to get the surprise.
For films on DVD pressing certain buttons at certain times will also show up some surprises.
Some of the Robbie Williams music CDs do have hidden last tracks that can be 30 minutes of silence then a poem or other comment from Robbie.