If you remember Super Mario Bros. 2, you know that there's a sort of subspace in the game. When you throw the beakers down on the ground, a door appears that temporarily leads to a shadow world. That world swaps the Super Mario Bros. 2 music out for the original Super Mario Bros. 1 overworld theme.

There's a glitch, though, that can bring that theme beyond the subspace and make it the music for Super Mario Bros. 2. That's what the video from YouTuber Gaijillionaire below demonstrates.

I never even knew this existed. Now I'm on my way to trying it on my trying it with my NES Classic.