When the player equips elytra and then unequips it by holding shift and clicking the destroy item button in the "Survival Inventory" tab of creative while holding a new pair of elytra with the cursor, the client and server desync, and the client believes the player is wearing elytra and thus should be able to fly but the server does not, as can be seen by using /data to check the player's inventory. This state persists even through gamemode changing, and by spamming space after switching to survival, the player can use this to jump much higher than usual. This does not work with normal items in normal inventory slots, but does seem to work with any item that can be put into the armor slots.