This is hard to explain and reproduce, but when you fly an elytra really fast and you move your mouse everywhere really fast, the legs of the player disappear and then you get kicked for an invalid received packet. Also if you are lucky enough, you glitch into the ground. Below, are two screenshots: One of them shows the bug in effect, with no legs. The other screenshot shows the kick message.

Edit: I forgot to mention that I am running an 18w06a Vanilla Server with disableElytraCheck set to true.

Edit2: I just realized you could face straight upwards to reproduce glitch.