The bug

If a player uses its elytra to fly into water and simultaneously tries to swim, it will have a buggy animation for other players around it and eventually freeze their clients so that they need to force the game to close and restart it.

To reproduce

1. Player A and Player B are on a server and can see each other
2. Player A activates their elytra and jumps into a deep enough water pond while pressing Ctrl (or whatever sprint is bound to)
3. Player B will observe Player A constantly switching between flight and swimming animation
4. Player B's client freezes eventually