Hackers find secret fifth coin
Friends, I know our culture of conspicuous consumption and wealth hoarding encourage us to collect all of the coins, but, alas, you will not be able to collect this coin. At least not without hacking like some sort of Russian.
The gist is that lines of coins on the ground are typically in groups of five, but this set of coins is on a hill, and one of the coins would otherwise clip too far beneath the ground because of the position of the coin spawner. Because of how deep into the ground it spawns, a fail safe unloads that coin (all five initially spawn together when Mario gets close enough) after noticing the coin spawner is too far beneath the ground.
The coin spawner is so low because the slope of the hill used to be different; it was changed to accommodate the big balls rolling down a different path, and the coin spawner was not moved up — this seems to be the only coin that triggers the unloading fail safe. Unless whoever wrote the fail safe just wanted it to be used at least once.