Sometimes I'll be listening to a tune and some other completely unrelated tune will start playing in my head over the top of it, even if I have to drastically change the rhythm or tempo to make it fit. 
 
Case in point. I was listening to Led Zeppelin's Kashmir last night:
 

 
...and it suddenly occurred to me that the theme from the UK 70's kids TV show The Magic Roundabout would sort of fit on the top of it, harmonically, if you slowed it down and changed the rhythm:
 

 
 And then I got to thinking about how fascinating the mind is when it comes to music. We must have hundreds of these harmonic blueprints stored away and cross referenced. 

I've got the Alphabet song and Twinkle Twinkle Little Star going through my head now.

I can barely keep one song in my head and now you want me to have two?

Commodore and The Cure
Easy Vs Just like and Heaven
https://vimeo.com/135195966

Anything by Rock Sugar.  These are great!  I liked them so much I bought the album. 
 

Only one pair...a very strange union.
 
Whenever I hear Darth Vader's theme in a Star War flick, about 4 or 5 bars into it (assuming 4 beats per measure), I find myself singing "...Makes the medicine go down, makes the medicine go down..." from the Mary Poppins tune "A Spoonful of Sugar". I can't help it...every time.

Be careful when experiencing this phenomenon:
 
https://www.gearslutz.com/board/11159666-post28.html
 
 

Kamikaze
Commodore and The Cure
Easy Vs Just like and Heaven
https://vimeo.com/135195966

Absolutely brilliant! Loved how this works.

It reminded me of another more quirky one from years ago:

https://www.youtube.com/watch?v=KTCYLbFxTpI