RARE fans will no doubt remember the much-trumpeted Stop 'N' Swop feature from Banjo-Kazooie. The original plan was for the feature to be used in Banjo-Kazooie and Banjo-Tooie in order to unlock new content between the games. Unfortunately, Nintendo squashed the idea before Banjo-Tooie came out, as the method to activate it was deemed too volatile.
RARE still had plans to include the Stop 'N' Swop feature in other games as well, including Donkey Kong 64. RARE actually sent out details to Nintendo on the feature, and Nintendo responded with a letter explaining why the idea wasn't really feasible. Nintendo found that it could cause problems with hardware, and wouldn't even work with all versions of the N64.
Nintendo also recommended that RARE find another way to make these features work, such as passwords that could be shared between games. RARE obviously didn't follow through with that idea, leaving Banjo-Kazooie's content and other plans for Stop 'N' Swop left to die.