Basically in Banjo 1, there were items like the Ice Key and some others that were clearly visible but impossible to get. Originally Rare intended to allow people to buy Banjo Tooie, unlock something, and then swap cartridges to go back and get the Ice Key.
Never materialized, although theories for how to use the "Stop 'n' Swop" persisted. What's interesting is that you can do something in BK to open up a Stop 'n' Swop menu, showing it was actually planned