Banjo-Kazooie shipped on June 29, 1998 with a mystery built into every cartridge. After collecting all 100 Jiggies in the game, Mumbo Jumbo would reveal the locations of 6 secret items: a collection of colored eggs and a single Ice Key, hidden in places most players never found. The game acknowledged these items. It showed you where they were. It gave you no way to use them.
That mystery turned out to be one of the most interesting stories about what happens when hardware changes mid-development.
What Stop 'n' Swop Was Supposed to Be
Rare designed a feature called Stop 'n' Swop to link Banjo-Kazooie directly to its sequel, Banjo-Tooie. The idea was straightforward: collect the hidden eggs and Ice Key in the first game, then unlock exclusive items or content in the second by carrying your progress across.
The transfer mechanism relied on how the Nintendo 64 handled memory at power-down. When you pulled a cartridge out of an N64 while the console was still on, the system briefly kept SRAM data powered -- roughly 8 to 10 seconds, depending on the unit. Rare calculated that was enough time for a player to remove the Banjo-Kazooie cartridge and insert Banjo-Tooie before that data window closed.
It was an elegant solution. No internet connection, no save-transfer cable, no special accessory. Just a fast swap and a small window of living memory.
Why It Never Shipped
Nintendo revised the N64's hardware during the console's production run. Later units reduced the SRAM power window to approximately 1 second after cartridge removal -- not nearly long enough to swap carts safely. Nintendo also raised a more practical concern: hot-swapping cartridges while the console remained powered risked damage to both the N64 and the game pak. A leaked internal Nintendo memo, later reported by gaming outlets, confirmed that Nintendo could not guarantee the RDRAM would retain data long enough for the feature to work reliably.