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
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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