When you are using an elytra and you smash into a wall the player will now be able to fit through 1 block gaps, it can happen both in survival mode and in creative, but it is harder to reproduce on creative. There are some ways to fix it.

To reproduce equip an elytra, go into survival mode and fly into a wall fast enough to receive damage and cancel the gliding (use a firework to do it easily), now you should be able to walk through 1 block gaps.

To fix you can sleep, or activate your elytra, or leave the world and join back, I was also able to fix it by swimming once, but I tried to do it again and it didnt work. 

I just realized the video I attached wont play here, I dont know why.