Way back in 1984, Christian rock band Prodigal hid a Commodore 64 program on their "Electric Eye" album. The 35-year-old Easter egg, which I won't spoil for you, was recently unlocked by YouTuber 8-Bit Show And Tell.
The son of band's lead singer, Dan Boldman, left this fun comment, "Loyd Boldman, the lead singer of Prodigal, was my dad & I thank you so much for doing this! This is the first time I've ever actually seen somebody use this program! I've been hearing about it since I was a kid!"
screenshot via 8-Bit Show and Tell