The bug

After using a firework to propel elytra flight, the firework boosting does not end after touching the ground. This causes firework particles to continue spawning underneath the player, and it means that the player will continue to be boosted if he starts flying again before the firework effect ends.

How to reproduce

1. Give yourself a firework with flight duration 50. (This can also be done with a flight duration of 3, which can be crafted with 3 gunpowder.)

   /give @p fireworks 1 0 {Fireworks:{Flight:50b}}

2. Glide with an elytra, use the firework, and quickly touch the ground.
3. Notice that the firework particles are still spawning below you. (They may appear inside the ground.)
4. Start gliding again. This will cause you to be immediately propelled by the firework.