The Unused Boss Key

This is actually the first time this mystery has been documented. In Super Mario 64 DS, you encounter Bowser, who drops a key in the first and second of his three fights - he drops what's officially known as a Jumbo Star in the third. You also encounter three new bosses that drop keys to the player's rooms. The thing is, the five keys are all one object, just with different object settings. But, there is actually a sixth key, which is known in a level editor - that the writer of this article uses - as a "Big Star Key".

Properties
In Super Mario 64, Mario is invited to Peach's Castle to eat a cake - which is shown in the ending. He hops out of a warp pipe, goes over to the castle, and enters it - after one of the Lakitu Bros. tells him the camera techniques. In the remake, this has been changed. Luigi and Wario come a long, and the three go to the castle. The same Lakitu wakes up Yoshi, who then starts searching for Mario and his buddies. There's one problem. The door is locked, and a rabbit has taken off with the key. Yoshi catches it, gets the key, and enters the castle. So what does the Big Star Key have to do with all this? Firstly, it uses the same model as the rabbit keys, but upscaled. Secondly and more importantly, it opens the same doors as the aforementioned rabbit key, although it acts like a normal boss key when you collect it.

Theories
The first theory is that there was an old boss that was scrapped. The doors were simply reused and probably recoded, and the developers forgot about the big star key. The second theory is that it's just used internally, given how it has similar properties to the key to the castle. A third theory holds that it's just an effect of how the keys work. It could also just be, as a fourth theory, an accidental creation by a developer.

Additional Notes
This mystery is a Mario Mysteries Wiki original! The writer actually found this mystery himself in 2019.