When the player glides faster than the speed of chunkloading, reaches an unloaded chunk and flies into terrain, when the world loads the player will sink 1 block into the ground and will be unable to land.

To reproduce:
1. Go to a high place, preferably y:255
2. Start gliding with an Elytra, go forwards at a small angle towards the ground so you accelerate
3. When the world around you isn't loaded, let yourself glide to ground level, y:64
4. When the chunk loading catches up, you'll be stuck in flight mode

It is possible that this bug can only be reproduced on slower computers, since it requires the player to actually fly faster than the speed of the world loading. I have successfully reproduced it 3 times on mine.