Server-client Desync Glitch (vertical fall example)
Whenever the server lags, the client's position might be offset server-side. If the position is greater than a half of a block, the server-side player will snap back to the client's position. However, since the server-side player also experiences world effects (like gravity and slime blocks), the player can enter many unintentional states.
Link to report: https://bugs.mojang.com/browse/MCPE-6...