In 1-3, there is an area that can be accessed by shooting through a wall when less than 50 seconds remains. The area has sweets in a pattern that reads "GAMES MASTER".

Games Master was a British television show in the late 1990's that reviewed video games, cheats, competitions and other game related material.
Contributed by Kakariko Kid
In the ending to the Amiga version, once Zool gets back to his planet there is a blue hedgehog with red shoes that walks across the screen before being kicked out of sight by Zool running by. This was meant to be a shot at Sega's Sonic the Hedgehog, which Zool was meant to be in direct competition with.
Contributed by gamemaster1991