I show that there's yet another impossible coin in the game, located in the huge version of Tiny-Huge Island. Specifically, there's a coin spawner there that's intended to spawn 5 coins in a horizontal line on the ground. However, this coin spawner's located under the ground, causing the most uphill coin to not load properly. In particular, this coin spawns about 49 units below the ground, triggering a failsafe that causes the coin to immediately unload. Currently, there's no known way to collect this coin.
Backstory: I had been documenting the various types of coin spawners for STROOP, since I was giving them each different names and images based on their subtype value. I had already created an image specifically for the coin spawner in THI, since I had fully expected it to have its own unique subtype for spawning exactly 4 coins, something that I knew no other coin spawner did. However, I was surprised to learn that its subtype matched that of a 5 coin spawner. I originally figured that there must be some other variable it had that allowed it to only spawn 4 coins instead of 5, but couldn't find such a variable after a quick search. The next day, it suddenly hit me: what if that coin spawner MEANT to spawn 5 coins, but we only saw 4 of them? It was the mystery goomba situation all over again. After doing more research, I found that this was in fact the case. However, I had expected this 5th coin to spawn on the death barrier way down below. But instead, the coin ended up unloading immediately after loading, a situation I didn't anticipate or understand the cause of. After having Peter look into the coin spawner's code, he explained the failsafe mechanism to me. Furthermore, he said that perhaps "part of the level got remodeled or something". This got me thinking, and after some more research, I fleshed out the hypothesis explained in the video.
The program I used in the video to show the object slots in real time is called STROOP. It's essentially an improved version of the SM64 diagnostic that I've shown previously. It's being developed primary by Dane Bouchie and myself, with help from Tyler Kehne, Peter Fedak, and Kaze Emanuar. It's not nearly done yet, but if you'd like to download what we have so far, you can do so from here: bit.ly/STROOPDownload
Additional Notes
(1) I had said that every coin line in the game has exactly 5 coins in it. However, some may try to point out exceptions to this, such as this group of coins in BitDW: i.imgur.com/21Lsdk2.png. However, formations such as that one are simply composed of multiple singleton coins placed adjacently. Those coins do not belong to a coin spawner, and do not load and unload as Mario goes close to and away from them (though they do disappear and reappear).
(2) Here's the image of the 4 coin line spawner that I had created in advance: i.imgur.com/4LmkTdM.png. I hadn't wanted it to go to waste, which is why I was so committed to determining whether the coin spawner in THI truly was a 4 coin line spawner or a 5 coin line spawner in disguise. After learning the truth, I learned that this image was never meant to be.