Karateka was the first game I saw and played on a friend's Apple II clone. It had a really cool easter egg (from the Wikipedia page) - The Apple II version cam

Karateka was the first game I saw and played on a friend's Apple II clone. It had a really cool easter egg (from the Wikipedia page) - The Apple II version came on one apparently single-sided disk. As an easter egg, a second version of the game was placed on the flip side of the disk. If one put the disk into the drive upside-down, the game played identically to the first side, except that the game was displayed vertically flipped. According to Mechner, this was done as a joke, causing naive users to call tech support and ask why the game was upside-down. Invariably, they would receive the reply, "take the disk out, insert it right-side up, and reboot".