Divorced Installment: According to comments from the developers at Blitz Games, this game started as a sequel to the previously released Frogger 2: Swampy's Revenge, which they made alongside publisher Infogrames / Hasbro Interactive under license from Konami. However, while they were working on an early version of a tentative Frogger 3, Konami became confident from the high sales of Frogger 2 that the series was still popular and decided to take back the license to make new Frogger games in-house and keep all the profits, resulting in the next game in the series being the poorly received Frogger: The Great Quest. Infogrames meanwhile still wanted to make a follow-up to Frogger 2 so they ordered Blitz to just remove all references to Frogger from the new game, leading to the end result of Zapper.
The GameCube version specifically contains some unused music left over from a prototype, including an extended version of the main menu theme.
There's an unused level theme for Voodoo Choo Choo, most likely intended for the second half of the level.
The game contains a hidden level called "Test 1". As its "name" suggests, it's little more than a test level, and was not intended to be playable. Interestingly, the level contains a texture of a developer's face that's seen nowhere else in the game.
There are five additional character names* Keith, Lugg, Ping, Lustre and Gadget contained within the game's text strings, all listed after Zapper's name; these are the names of the multicoloured crickets you meet scattered across the game's levels, and were visible in the cheats and extras menu in the July 2002 prototype of the game.
Keep Circulating the Tapes: The game received a release on Xbox Live's Games on Demand service in 2008. For a long time this was the only official re-release it had, and any potential future re-releases were doubtful with Blitz Games' closure in 2013 and Atari's apathy towards much of their later Infogrames-era output. It was finally averted in February 2024 when Atari re-released the PC version through Steam and GOG.
What Could Have Been: In March 2021, a prototype of the game dated from just two months before it was finished was leaked, containing content that didn't make it to the final game:
An entire world taking place on an oil rig was made but hastily removed, which would have raised the total amount of worlds from four to five and the amount of levels from seventeen to twenty. As a result, the level order was re-arranged slightly: Pipewerx, originally the final level of the cut world (not including bonus levels), was now inserted between Laser Maze and Chain Reactor, while Hasta la Vista Magpie (the boss level, and originally the final level of World Five, which became World Four in the final game) was repurposed as a stand-alone level. Notably, the GBA version, developed by Atomic Planet, contains an adaptation of this area dropped from the console/PC version.
The other crickets that give you hints throughout the game would have been unlockable playable characters to use in the normal levels, while in the final game they are only playable in the multiplayer mode.
Voodoo Choo Choo had more elaborate background scenery, which was reduced to a simple looping background in the final game.
Zapper's chirp, which remains in the console versions merely as a Taunt Button and was removed entirely from the PC version, was supposed to be used to light up dark areas. Zapper automatically lights up dark areas with his antennae in the final product, though the text strings referencing this cut ability were left in.
There's unused main menu and credits themes. The former of which according to composer Matt Black was "the first go I had at the menu music and it was rejected by our producers at the time. I think they thought it was maybe too much like a Spy/Bond theme and not "fun" enough. However I still really liked parts of it, so thought it would be cool to slip it in as part of a level music. Which is why it features there for the Laser Maze music." The latter while not specified was likely rejected for the same reason since it's just a slightly different reprise of the former.
