Musical Necker cubes
The simplest and most effective optical illusion ever is the Necker cube. Which side is in front? The answer is both and neither. Very Zen.
In the process of gathering musical simples, I found a P-Funk loop with a similar effect. It's a keyboard lick from "Do That Stuff" from The Clones Of Dr Funkenstein.
This loop is best known to the hip-hop generation as the basis of "Funky For You" by Nice & Smooth. Greg Nice begins his first verse halfway through the loop. Apparently he hears the E chord as being the beginning of the loop.
I had heard "Do That Stuff" many times before ever hearing "Funky For You," so to me, Greg Nice seems to be starting in the wrong spot. However, when I posted my transcription of the loop, I heard back that I had it backwards. If you heard Greg Nice before P-Funk, you probably agree with his interpretation.
What is it about the loop that makes its front and back halves so interchangeable? Usually these kinds of phrases start on the tonic chord, the root, harmonic "home base." The tonic chord of "Do That Stuff" is E, but in the loop, the E chord is in the back half of the loop, which is a metrically weak spot. If you walked into the studio while the loop was playing, you might well intuit that it had the tonic at the beginning of the loop as would be more usual.
"Get Lucky" by Daft Punk is based on a similarly ambiguous pattern. It's also a four-chord loop: B minor, D, F# minor, E. It isn't at all clear which of these four chords is the tonic. I hear it as F# minor, which is in the same metrically weak location as the tonic in the "Do That Stuff" loop. If you used the "Get Lucky" loop as a hip-hop sample, and your emcee wasn't familiar with the original song, they'd have a fifty-fifty chance of coming in on the third chord.
The other musical Necker cube that leaps to mind is the omnipresent I-V-vi-IV progression. It can be flipped around to form the equally plausible vi-IV-I-V progression, as heard in the later part of the "Four Chords, Thirty-Six Songs" video.
https://www.youtube.com/watch?v=i4_f6pfabQk
If you can think of any other musical Necker cubes, post them in the comments.