Karateka (Apple II)
This game has hidden bonus content.
Karateka is a rotoscoped side-scrolling brawler in which a stiff-legged karate man kicks everything. Even birds.
While Karateka only takes up Side A, Side B contains a copy of the game modified to play upside-down. According to Jordan Mechner, this was a practical joke by Brøderbund so tech support could have the delight of telling someone that their game was upside-down because...they put the disc in upside-down. Duh.
The Karateka series