Supper Mario Broth is excellent! You may already know of this Mario esoterica blog and its prodigious post rate. Somehow they keep finding interesting things to publish!
Here’s a recent mindblower. The four note sound effect from picking up a Red Coin in Super Mario 64 is actually a brief segment of the Bob-Omb Battlefield theme, pitch-shifted according to how many Red Coins you’ve found so far. Their post on the phenomenon has the sound synced up to match the point in the music it’s taken from, and it’s unmistakable once you hear it!