Jet Force Gemini is a sci-fi action adventure / third-person shooter developed by Rare that was released in 1999 for the Nintendo 64. The main characters, Juno and Vela, were originally designed as younger in the beta version and their 3D models were more cartoon-like with large heads. They were later changed to their more mature versions before the release of the game. However, a similar set of childish models can still be played with via an in-game cheat.
Jet Force Gemini does not support the 4MB Expansion Pak, although Rare had considered the possibility. This led to some confusion as the box cover for the original release stated that it did support the Expansion Pak. Nintendo provided a quick-fix to the mislabeled covers by providing stickers declaring its rumble pak compatibility and fixed later printings of the boxes. [Info from Wikipedia]
Also, some debug rooms and beta maps were found still hidden in the game thanks to ps3punk7890, Luigi 316 and SubDrag. An interesting note, one of these unused maps is a racing track from Diddy Kong Racing: it’s possible that they used this to test the racing missions in Jet Force Gemini in its early stages.
Jet Force Gemini Beta