In a multiplayer world, double right click quickly on a boat.

The server thinks you are in a boat, but the client can walk around in a small radius ( a couple chunks )

After that, the collision doesn't work and you fall straight trough the world

Hostile mobs still think you are in the boat and attack you in it ( you still take damage )

You can't take any fall damage when in this glitched state

Blocks also glitch when placed a little further away from the boat

Sneaking teleports you back to the boat and stops the glitch.

Attachment 1 and 2: glitched scaffolding blocks
Attachment 3 and 4: skeleton doesn't spot me, even in survival mode