This bug lets you to use a bit of redstone and Elytra to change your hitbox to the flying mode hitbox and crawl in small spaces.

See video - https://goo.gl/ADIQdX

Pros: You still have normal movement

Cons: Requires elytra, shifting will cause suffocation, and your hitbox will revert back to normal once you go into a two block tall space

How the contraption works: Normally, flying into a one block space is impossible because it pushes you back out. The contraption closes the space above you using a piston which closes the gap above you, preventing you from standing normally and forcing the character to crawl.