You might think we know everything there is to know about Super Mario 64, but that's not the case. The game continues to give up its secrets, and the latest involves a coin which is impossible to collect.
The N64 classic actually has a handful of coins that cannot be obtained by the player, but the one in question - found in the huge version of Tiny-Huge Island - appears to be the product of a late change to the level design.
Coins spawn in Super Mario 64 in a variety of ways, but they're always in groups of five. In this case, there's a line of four coins which suggests that one is missing.
According to YouTuber UncommentatedPannen, the game determines where to place coins by looking at the distance between where the coins spawn and the ground. In this instance the spawn spot is so far below the level of the ground that the coin appears below the slope, thereby triggering a failsafe that forces it to vanish.
Why does this happen? UncommentatedPannen believes that the coin disappears because it was placed and then the angle of the slope was altered, pushing the spawn point below the ground. The developers didn't reposition the coin line, and the result is an "impossible coin" which should be there to collect, but isn't.
Super Mario 64: The gift that keeps on giving.