Pink Floyd were always into audio tricks and secret messages. Original copies of their Atom Heart Mother album ended with the sound of a dripping tap set on a locked groove, meaning you'd keep hearing the dripping tap till you got up and lifted the needle off the record.
Grantchester Meadows ended with the sound of a buzzing fly panning back and forth between the left and right channels which really messes with people listening with headphones.
The 8-track version of Pigs on the Wing had an added guitar solo that linked the two parts of the song together and accomodated the fact 8-track tapes play in a continous loop until they're removed from the tape deck.
Roger Waters hid a couple of secret messages on his solo albums, on Radio Kaos there's Morse code at the end of the final track, The Tide is Turning that reveals a verse cut from the lyrics.
On Perfect Sense Part 1 off Amused to Death there's a backwards message referencing that Roger wanted the song to start with a sample from 2001 A Space Odyssey but Kubrick didn't allow it. Kubrick was having revenge because Pink Floyd turned down his request to use part of title track from Atom Heart Mother in A Clockwork Orange.
My copy of "Sabrina the Teenage Witch: The Album" (Geffen Records) had Sabrina's family secret of becoming a teenage witch (although she is portrayed by none other than Melissa Joan Hart) at the end of the CD
Lisa Frank - You Gotta Have It! Pokemon - Gotta Catch 'Em All!