When I was a kid, the Ice Key in Banjo-Kazooie drove me insane! The way you could see it but never reach it. That and the closed tomb in the desert level and the closed wine barrel in the haunted mansion. And goddamn Mumbo Jumbo just taunted me with those pictures after you 100% the game
I just googled it and apparently you can access all these things by entering ridiculously long cheatcodes in the sandcastle in the pirate level.
Apparently the ice key does nothing. When you get it Kazooie says "maybe we should hold onto this!" but then it does nothing. And you get eggs for the other things (as well as a few others).
The key and eggs were intended to be transferred to Tooie somehow. Rare had a plan to use the RAM of the N64 as a way to transfer data between the two games (when a new game is loaded, the RAM initially contains whatever was previously in it - so if they made sure Kazooie wrote the right stuff before it was shut off, Tooie could check the contents before clearing room for itself). Apparently Nintendo didn't like the idea so it was cut at the last second.
So in Tooie there's an ice chest which you need the ice key to open. Since the transferring was cut, the devs added the ice key and eggs to Tooie. I remember renting Tooie as a kid and getting the ice key, though I don't remember how... I barely remember that game and I never beat it. Now I want to play it again!
I remember most of Banjo-Kazooie. It was one of my favorite N64 games as a kid.